[Infra] Use Bazel 6 and drop Bazel 4 (#32712)

Let's move on as we'd agreed to drop 4.x

bazel/supported_versions.txt

@@ -1,3 +1,2 @@
-6.0.0
+6.1.1
 5.4.0
-4.2.3

doc/bazel_support.md

@@ -39,6 +39,5 @@ recent LTS versions of Bazel. However individual releases may have a broader
 compatibility range. The currently supported versions are captured by the
 following list:
 
-- [`6.0.0`](https://github.com/bazelbuild/bazel/releases/tag/6.0.0)
+- [`6.1.1`](https://github.com/bazelbuild/bazel/releases/tag/6.1.1)
 - [`5.4.0`](https://github.com/bazelbuild/bazel/releases/tag/5.4.0)
-- [`4.2.3`](https://github.com/bazelbuild/bazel/releases/tag/4.2.3)