Drop Built-Date, Built-JDK, Created-By from plugin manifests (#7578)

`new Date()` was evaluated at configuration time on every Gradle
invocation, so each plugin jar's manifest embedded the build wall-clock.
That made the jars non-deterministic byte-for-byte across builds and
broke Gradle's configuration cache for plugin tasks.

The companion `Built-JDK` and `Created-By` attributes were dropped at
the same time because:

1. Nothing in the codebase reads any of these three attributes
   (`grep -rn 'Built-Date|Built-JDK|Created-By'` returns only the writer
   line). They were inert metadata.
2. The main `pcgen.jar` manifest does not carry equivalents, so the
   plugin jars were the odd ones out.
3. Real reproducibility would also require pinning archive timestamps
   (`preserveFileTimestamps = false`), file ordering
   (`reproducibleFileOrder = true`), and the JDK/Gradle versions across
   environments — for *all* archive tasks in the project, not just the
   plugin jars. Keeping `Built-JDK`/`Created-By` on plugin jars only
   would be inconsistent: it would leak environment info from a subset
   of artifacts without buying real reproducibility. If a future PR
   pursues full reproducible builds, it can apply those settings
   project-wide and re-introduce diagnostic attributes uniformly.

Verified: two clean builds of `:jarExportPlugins` now produce
byte-identical jars; configuration cache stores successfully for
`:jarAllPlugins`.

Author: Vest

code/gradle/plugins.gradle

@@ -26,10 +26,7 @@ def createJarTask = {taskName, archiveName, description, includePattern ->
             attributes(
                     "Implementation-Title": "PCGen ${taskName.replace('jar', '').toLowerCase()} plugins",
                     "Implementation-Version": project.version,
-                    "Class-Path": "lib/pcgen.jar",
-                    "Built-Date": new Date(),
-                    "Built-JDK": "${System.getProperty('java.vm.version')} (${System.getProperty('java.vendor')})",
-                    "Created-By": "Gradle ${project.gradle.gradleVersion}"
+                    "Class-Path": "lib/pcgen.jar"
             )
         }