changelog: add missing PR links to v1.12.0 entries

[denik]

Eight entries in the v1.12.0 section shipped without a PR link. The numbers are
recovered from the commits that added the corresponding .nextchanges fragments.

The Terraform bump entry now links both PRs it covers, since
#6250 replaced the fragment from
#6246.

[github-actions]

Waiting for approval

Based on git history, these people are best suited to review:

• @andrewnester -- recent work in ./
• @shreyas-goenka -- recent work in ./

Eligible reviewers: @anton-107, @janniklasrose, @lennartkats-db, @pietern, @rclarey, @renaudhartert-db, @rugpanov, @simonfaltum

_{Suggestions based on git history. See OWNERS for ownership rules.}