[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

notes/2.0.0/cache-version.md

@@ -0,0 +1,24 @@
+## cacheVersion setting
+
+sbt 2.x caches task results by default. `cacheVersion` provides an escape hatch to invalidate all caches when needed.
+
+### Usage
+
+In `build.sbt`:
+
+```scala
+Global / cacheVersion := 1L
+```
+
+Or via system property:
+
+```
+sbt -Dsbt.cacheversion=1
+```
+
+### Details
+
+- Defaults to reading system property `sbt.cacheversion`, or else `0L`
+- When `cacheVersion` is `0L`, caching behaves identically to previous versions
+- Changing the value invalidates all task caches, forcing recomputation
+- The value is incorporated into every cache key via `BuildWideCacheConfiguration`