[2.x] feat: Add cacheVersion setting for global cache invalidation (#8993)

* [2.x] feat: Add cacheVersion setting for global cache invalidation

**Problem**
There was no escape hatch to invalidate all task caches when needed.

**Solution**
Add `Global / cacheVersion` setting that incorporates into the cache key
hash. Changing it invalidates all caches. Defaults to reading system
property `sbt.cacheversion`, or else 0L. When 0L, the hash is identical
to the previous behavior (backward compatible).

Fixes #8992

* [2.x] refactor: Simplify BuildWideCacheConfiguration and add cacheVersion test

- Replace auxiliary constructors with default parameter values
- Add unit test verifying cacheVersion invalidates the cache

* [2.x] fix: Restore auxiliary constructors for binary compatibility

* [2.x] test: Improve cacheVersion scripted test and add release note

- Scripted test now verifies cache invalidation via a counter
  that increments only when the task body actually executes
- Add release note documenting the cacheVersion setting

main-command/src/main/scala/sbt/BasicKeys.scala

@@ -129,6 +129,12 @@ object BasicKeys {
     10000
   )
 
+  val cacheVersion = AttributeKey[Long](
+    "cacheVersion",
+    "A version number that invalidates all task caches when changed.",
+    10000
+  )
+
   // Unlike other BasicKeys, this is not used directly as a setting key,
   // and severLog / logLevel is used instead.
   private[sbt] val serverLogLevel =