docs: Announce Java 11 and minSdk 26 requirements for SDK 9 (#6471)

* docs: Announce Java 11 and minSdk 26 requirements for SDK 9

Add a warning at the top of the changelog that SDK 9.0.0 drops Java 8
and raises the Android minSdk from 21 to 26, linking to the feedback
issues so users can raise concerns before the major release.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* docs: Drop the stay-on-8.x line from the SDK 9 warning

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: runningcode

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+> [!WARNING]
+> SDK 9.0.0 will raise the minimum supported Java and Android versions:
+> - Java 8 support will be dropped. SDK 9.0.0 requires Java 11+. See [#6469](https://github.com/getsentry/sentry-java/issues/6469).
+> - The Android `minSdk` will be raised from 21 to 26. See [#5508](https://github.com/getsentry/sentry-java/issues/5508).
+
 ## 8.63.0
 
 ### Features