feat: widen urbanairship-core dependency to any 20.x

Replace the pinned `urbanairship-core:20.3.0` with a bounded range
`[20.3.0,21.0.0)` so consumers can opt into newer 20.x versions
(20.4.0–20.7.x available today) without waiting for a kit release.

The closed upper bound at 21.0.0 keeps a future Airship major out of
the resolved set, so consumers explicitly opt in to 21.x via a new
kit release.

Consumers who want a specific 20.x can pin it in their own app:

    implementation 'com.urbanairship.android:urbanairship-core:20.6.0'

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jamesnrokt

build.gradle

@@ -61,7 +61,8 @@ android {
 
 dependencies {
     compileOnly 'androidx.legacy:legacy-support-v4:1.0.0'
-    api 'com.urbanairship.android:urbanairship-core:20.3.0'
+    // Allow any 20.x; consumers can override to a specific 20.x in their app build.
+    api 'com.urbanairship.android:urbanairship-core:[20.3.0,21.0.0)'
     testImplementation 'junit:junit:4.13.2'
     testImplementation files('libs/java-json.jar')
 }