build(deps): Upgrade default Kotlin to 2.1.21 (#1265)

Bump the default Kotlin version from 1.8.20 to the latest 2.1.x release.
Staying on 2.1.x keeps the plugin-build language version at 1.8: that
build derives languageVersion/apiVersion from the oldest the compiler
still supports (current minus three), which is 1.8 for Kotlin 2.1 but
2.0 for Kotlin 2.3. A note on Dependencies.kt records this so future
bumps are aware they may raise the language level.

KSP1 is patch-coupled to the Kotlin compiler, so the default KSP1
fallback is bumped from 2.1.0-1.0.29 to 2.1.21-2.0.2 to match; the old
one crashed the KSP compile with an internal compiler error on 2.1.21.

Make BuildPluginsVersion.KOTLIN the single source of truth: the kotlin
plugin aliases in the catalog are now versionless and take their version
from the apply site. The standalone sentry-snapshots-runtime included
build has no buildSrc and can't reach Dependencies.kt, so it (and the
kotlinJunit test dependency) pin the version inline.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: runningcode

buildSrc/src/main/java/Dependencies.kt

@@ -1,14 +1,17 @@
 object BuildPluginsVersion {
     val AGP = System.getenv("VERSION_AGP") ?: "8.10.1"
-    val KOTLIN = System.getenv("VERSION_KOTLIN") ?: "1.8.20"
+    // Bumping this may implicitly raise the language level: plugin-build derives its
+    // languageVersion/apiVersion from the oldest the compiler still supports (current minus
+    // three, floored at 1.8), so e.g. Kotlin 2.1 keeps 1.8 but Kotlin 2.3 forces 2.0.
+    val KOTLIN = System.getenv("VERSION_KOTLIN") ?: "2.1.21"
     // KSP1 (X.Y.Z-A.B.C) is bound to a specific Kotlin compiler version; KSP2 (e.g. 2.3.7) is
     // decoupled and supports Kotlin language version 2.0+. Default to KSP1 for the default
-    // Kotlin 1.8.20, and switch to KSP2 when the matrix sets a Kotlin 2.x version.
+    // Kotlin 2.1.21, and switch to KSP2 when the matrix sets a Kotlin 2.x version.
     val KSP = System.getenv("VERSION_KOTLIN")
         ?.substringBefore('.')
         ?.toIntOrNull()
         ?.let { if (it >= 2) "2.3.7" else null }
-        ?: "2.1.0-1.0.29"
+        ?: "2.1.21-2.0.2"
 }
 
 object LibsVersion {

gradle/libs.versions.toml

@@ -1,5 +1,4 @@
 [versions]
-kotlin = "1.8.20"
 agp = "8.10.1"
 
 asm = "9.4" # // compatibility matrix -> https://developer.android.com/reference/tools/gradle-api/7.1/com/android/build/api/instrumentation/InstrumentationContext#apiversion
@@ -17,10 +16,11 @@ sampleRetrofit = "2.9.0"
 sampleSpringBoot = "3.4.1"
 
 [plugins]
-kotlin = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
-kotlinAndroid = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
-kotlinSpring = { id = "org.jetbrains.kotlin.plugin.spring", version.ref = "kotlin" }
-kapt = { id = "org.jetbrains.kotlin.kapt", version.ref = "kotlin" }
+# Version is supplied at the apply site, normally from BuildPluginsVersion.KOTLIN.
+kotlin = { id = "org.jetbrains.kotlin.jvm" }
+kotlinAndroid = { id = "org.jetbrains.kotlin.android" }
+kotlinSpring = { id = "org.jetbrains.kotlin.plugin.spring" }
+kapt = { id = "org.jetbrains.kotlin.kapt" }
 ksp = { id = "com.google.devtools.ksp" }
 dokka = { id = "org.jetbrains.dokka", version = "1.9.20" }
 spotless = { id = "com.diffplug.spotless", version = "8.5.0" }
@@ -43,7 +43,7 @@ agp = { group = "com.android.tools.build", name = "gradle", version.ref = "agp"
 kotlinCompilerEmbeddable = { group = "org.jetbrains.kotlin", name = "kotlin-compiler-embeddable" }
 autoService = { group = "com.google.auto.service", name = "auto-service", version = "1.1.1" }
 autoServiceAnnotatons = { group = "com.google.auto.service", name = "auto-service-annotations", version = "1.1.1" }
-kotlinJunit = { group = "org.jetbrains.kotlin", name = "kotlin-test-junit", version.ref = "kotlin" }
+kotlinJunit = { group = "org.jetbrains.kotlin", name = "kotlin-test-junit", version = "2.1.21" }
 kotlinCompileTesting = { group = "dev.zacsweers.kctfork", name = "core", version = "0.13.0" }
 truth = { module = "com.google.truth:truth", version = "1.4.5" }
 composeDesktop = { group = "org.jetbrains.compose.desktop", name = "desktop", version = "1.6.10" }

sentry-snapshots-runtime/build.gradle.kts

@@ -1,5 +1,6 @@
 plugins {
-  alias(libs.plugins.kotlin)
+  // No buildSrc here, so this standalone included build can't read BuildPluginsVersion.KOTLIN.
+  alias(libs.plugins.kotlin) version "2.1.21"
   id("distribution")
   alias(libs.plugins.mavenPublish)
   alias(libs.plugins.spotless)