UI text: filters should have consistent spelling

Search filters for issues or MRs have inconsistent spelling.

They should all follow our guidance for button labels: use sentence case and spaces for word separators.

Implementation plan

The following search filters should be updated:

• My-Reaction to My reaction
  • !185235 (merged)
• Search Within to Search within
  • !185242 (merged)
• Approved-By to Approved by (on MR search)
  • !187159 (merged)
• Merged-By to Merged by (on MR search)
  • !187159 (merged)
• Deployed-after to Deployed after (on MR search)
  • !187159 (merged)
• Deployed-before to Deployed before (on MR search)
  • !187159 (merged)
• Target Branch to Target branch (on MR search)
  • !186160 (merged)
• Source Branch to Source branch (on MR search)
  • !186160 (merged)

When editing the spellings, remember to:

• Update the filter names in docs if needed. No need for a history item.
• Fix related spec files.
• Regenerate gitlab.pot with tooling/bin/gettext_extractor locale/gitlab.pot.