Ruby: define Google::Protobuf::VERSION in lib/google/protobuf/version.rb (#30762)

### Motivation

The `google-protobuf` gem does not expose its version in Ruby code. The version is only a string literal in `ruby/google-protobuf.gemspec`. Most gems define a `VERSION` constant in `lib/<gem>/version.rb`, and the gemspec reads it from there. This lets applications and tools read the version at runtime with `Google::Protobuf::VERSION`.

### Changes

- Add `ruby/lib/google/protobuf/version.rb`, which defines `Google::Protobuf::VERSION`.
- Make `ruby/google-protobuf.gemspec` read `s.version` from `Google::Protobuf::VERSION`.
- Require `google/protobuf/version` at the end of `ruby/lib/google/protobuf.rb`, so the constant is available after `require "google/protobuf"`.
- Add `//ruby/lib/google:protobuf/version.rb` to the `rb_bundle_fetch` sources in `WORKSPACE`, because the gemspec now requires the file.

### Required change to the internal version update tool

> [!IMPORTANT]
> The internal version update tool (the `update_versions` library that makes the "Updating version.json and repo version numbers" commits) must change after this PR merges.

The tool currently writes the Ruby version into `ruby/google-protobuf.gemspec`. The legacy `update_version.py` (removed in #9983) used this pattern:

```python
RewriteTextFile('ruby/google-protobuf.gemspec',
  lambda line : re.sub(
    r'^  s.version     = ".*"$',
    '  s.version     = "%s"' % GetFullVersion(rc_suffix = '.rc.'),
    line))
```

After this PR, the gemspec does not contain a version literal, so that pattern matches nothing. The tool must write the version into `ruby/lib/google/protobuf/version.rb` instead, for example:

```python
RewriteTextFile('ruby/lib/google/protobuf/version.rb',
  lambda line : re.sub(
    r'^    VERSION = ".*"$',
    '    VERSION = "%s"' % GetFullVersion(rc_suffix = '.rc.'),
    line))
```

If the tool does not change, the next version bump will leave `version.rb` at the old version without an error, and the gem will be published with the wrong version. `PROTOBUF_RUBY_VERSION` in `protobuf_version.bzl` still names the `.gem` file in the Bazel release rules, so the tool must continue to update both values.

Closes #30762

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/30762 from Shopify:uk-add-version-file 36692bbd9ae1561d54622086f4c0e56ddb69fab1
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