Bump versions of Bazel and some deps.

This was prompted by the fact that my build used Bazel 8.0.0 when I didn't want it during cl/706839076 because I was running an [old](bazelbuild/bazelisk#502) version of Bazelisk. I fixed that by upgrading Bazelisk, but I figured I might as well update Bazel, too.

Along the way, I noticed this:

```
WARNING: For repository 'rules_java', the root module requires module version rules_java@7.12.1, but got rules_java@8.6.1 in the resolved dependency graph. Please update the version in your MODULE.bazel or set --check_direct_dependencies=off
```

So I upgraded `rules_java`, too, and `rules_jvm_external` while I was in the area.

RELNOTES=n/a
PiperOrigin-RevId: 707159453

.bazelversion

@@ -1 +1 @@
-7.3.1
+8.0.0

MODULE.bazel

@@ -15,8 +15,8 @@
 module(name = "google_bazel_common")
 
 bazel_dep(name = "bazel_skylib", version = "1.7.1")
-bazel_dep(name = "rules_java", version = "7.12.1")
-bazel_dep(name = "rules_jvm_external", version = "6.4")
+bazel_dep(name = "rules_java", version = "8.6.1")
+bazel_dep(name = "rules_jvm_external", version = "6.6")
 
 GUAVA_VERSION = "33.4.0-jre"