feat(presets): add changelogUrl link for GitLab-based digest updates

[RahulGautamSingh]

Changes

Add a gitlabDigestChangelogs helper preset that provides a link to the commit-to-commit diff for dependencies pinned by digest and sourced from GitLab.com. This mirrors the existing githubDigestChangelogs preset, but targets gitlab-releases, gitlab-tags, git-refs, and git-tags datasources with https://gitlab.com/** source URLs.

The preset is also included in config:recommended so users get GitLab.com changelog links out of the box.

Context

• This closes an existing Issue, feat(presets): add changelogUrl link for GitLab-based digest updates #39732
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI generated non-trivial portions of code, tests, or documentation).

Documentation (please check one with an [x])

• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Newly added/modified unit tests