Removing obsolete GH Action & Correcting Dependabot

## The assignment

Removing obsolete GH Action dependent-issues.yml, it's now part of Github: https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/creating-issue-dependencies#marking-an-issue-as-blocked-by-or-blocking-another-issue

https://github.blog/changelog/2025-08-21-dependencies-on-issues/

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Removing as SBT is not supported by Dependabot yet.
