chore: Yet more string cleanup

Description

Follow-up to !1570 (merged) that cleans Yet More Strings. Still not done, but this MR tackles issuables which was a lot.

grep -riI "fmt.Errorf(" . has been helpful in finding problems.

Related Issues

• Related to Track holistic UI string cleanup (#7598 - closed) where I'm tracking the Technical Writing work
• Related to chore: Fix capitalization and punctuation in co... (!1570 - merged)
• Follow-up to chore: Fix punctuation and phrasing in messages (!1566 - merged)
• Related to feat(stacked-diffs): Create initial sync command (!1517 - merged) where Tomas reminded me to do string checks: !1517 (comment 1951010899)
• Related to Add stacked diff syncing to merge requests (#7470 - closed) which started all this work

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap