Fixes the following Rocky Linux 8 Bazel release job failure described in
bazelbuild#27463 (comment):

```txt
ERROR: Skipping ':maven-srcs': no such target '//:maven-srcs':
  target 'maven-srcs' not declared in package ''
  defined by /workdir/BUILD
WARNING: Target pattern parsing failed.
ERROR: no such target '//:maven-srcs':
  target 'maven-srcs' not declared in package ''
  defined by /workdir/BUILD
```

Reproduced and confirmed the fix by executing the failing release job
command locally:

```txt
./bazel-bin/src/bazel build --nobuild \
  :bazel-srcs :bootstrap-jars :maven-srcs //src:derived_java_srcs
```

Part of bazelbuild#28177.

---

Background from:

- bazelbuild#27463 (comment)

`bazelbuild/continuous-integration/pipelines/bazel-release.yml` defines
the failing Rocky Linux 8 job. (This was a CentOS 7 job prior to
bazelbuild/continuous-integration#2281.)

- https://github.com/bazelbuild/continuous-integration/blob/agent-0.2.7/pipelines/bazel-release.yml#L85

bazelbuild/bazel@3d7937e introduced
the `//:maven-srcs` target in Bazel 7.0.0. All the other targets in the
failing `bazel build` invocation exist on the Bazel 6.6.0 branch.

bazelbuild/continuous-integration@14a0378
added the `//:maven-srcs` target to this job as an attempt to fix
`MODULE.bazel.lock` generation.

Neither the lock file nor the `//:maven-srcs` target itself should have
any bearing on the Bazel 6.6.0 release. This dummy target fixes the
build failure and allows the release to proceed.