Support get changelog from Self-Managed Gitlab

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Under the npm ecosystem, there are some private dependencies configured `repository` filed on `package.json`

```json
{
"name": "@private/package",
"version": "14.5.0",
"repository": {
"type": "git",
"url": "https://self-managed-gitlab-URL.com/group-name/repo-name.git",
}
}
```

https://github.com/dependabot/dependabot-core/blob/63977df77903e646c52f88eef803b29c4f805fd2/npm_and_yarn/lib/dependabot/npm_and_yarn/metadata_finder.rb#L71-L81

So dependabot can get `url` correctly.

https://github.com/dependabot/dependabot-core/blob/63977df77903e646c52f88eef803b29c4f805fd2/common/lib/dependabot/source.rb#L67-L70

But `self-managed-gitlab-URL` can't be matched

https://github.com/dependabot/dependabot-core/blob/63977df77903e646c52f88eef803b29c4f805fd2/common/lib/dependabot/metadata_finders/base/changelog_finder.rb#L162-L176

As the result, dependabot can't add changelog at pr description


Start with common/lib/dependabot/source.rb and common/lib/dependabot/metadata_finders/base/changelog_finder.rb, then compare how npm_and_yarn/lib/dependabot/npm_and_yarn/metadata_finder.rb obtains the repository URL. Confirm the self-managed GitLab URL is recognized and that the generated update PR description includes the dependency changelog.

Written by the indexing model from the issue text.