fix: harden release note generation

[mattkjames7]

A recent release workflow run failed due to the title of this PR containing issue number it resolved. The resulting commit that gets parsed for PR numbers is titled: fix: prefix mg-exporter resources with release fullname (#233) (#261). The script for building the changelog items matched both numbers in the title and failed to find the PR #233. This fix should match only the last number found in a title and also handle the 404 received when it cannot fetch the PR.

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[mattkjames7]

Tracking

• [Link to Epic/Issue]

Standard development

• Update / add chart templates, values, and NOTES.txt as needed
• Run helm lint and helm template locally against the changed chart(s)
• Verify the chart installs and upgrades cleanly on a test cluster

Labels checklist

• Add a docs label (exactly one): docs-changelog-only, docs-needed, or docs-not-needed
• Add at least one component label: memgraph, memgraph-ha, memgraph-lab, or infrastructure
• Add at least one type label: bug, feature, or infrastructure
• Assign the PR to a milestone
  • If not known, set for a later milestone

Documentation checklist

• Write a release note, including added/changed clauses
  • N/A
• [ Documentation PR link memgraph/documentation#XXXX ]
  • Is back linked to this development PR