Require auto-merge and rebase for Dependabot (#49)

[mattfsourcecode]

This PR contributes to #49 by enforcing auto-merge as a required check for Dependabot PRs and ensuring that they are always rebased before merging.

Why This Change Is Needed

Previously, gh pr merge --auto --merge "$PR_URL" failed with:

GraphQL: Base branch was modified. Review and try the merge again. (mergePullRequest)
Error: Process completed with exit code 1.

https://github.com/mattfsourcecode/fastify-swc-typescript-server/actions/runs/13182494086/job/36796536875

This issue occurred when the base branch was modified by update-licenses after Dependabot created its PR, preventing automatic merging.

Key Changes:

• Adds automatic rebase for Dependabot PRs to ensure they are always up to date before merging.
• Makes auto-merge a required status check for Dependabot PRs.
• Prevents auto-merge from affecting non-Dependabot PRs, ensuring normal PRs are not blocked.

[mattfsourcecode]

Additionally, auto-merge has been added as a required status check in the project rulesets. This merge will allow the auto-merge check to exit for non-Dependabot PRs.