Rename maven to protobuf_maven in MODULE.bazel (#18641)

The default name for the rules_jvm_external.maven rule is "maven". When not set, it defaults to "maven". For root modules also using rules_jvm_external, the name clash causes a warning:

```
DEBUG: $TMP/external/rules_jvm_external~/private/extensions/maven.bzl:154:14:
The maven repository 'maven' is used in two different bazel modules,
originally in '<my_workspace>' and now in 'protobuf'
```

Summarizing @shs96c in [1]:

> The common maven repo name allows rulesets to contribute to the user's JARs.
> However, this implies that maven is for the end user, not for transitive
> dependencies. If a ruleset needs private dependencies, it should use a custom
> namespace rather than the maven namespace.

Since protobuf is not contributing to user's JARs, we'll use a custom namespace. There's precedent for using a custom namespace for library modules:

- rules_jvm_external uses `rules_jvm_external_deps` instead of `maven`.
- rules_kotlin uses `kotlin_rules_maven` instead of `maven`.

[1]: https://github.com/bazel-contrib/rules_jvm_external/issues/916#issuecomment-1645527584

Fixes #16839.

Closes #18641

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/18641 from jschaf:joe/protobuf-maven bd2c62f3118264adc1e725102d1ecc576f145f35
PiperOrigin-RevId: 684625084