Standardize merge details messaging

What does this MR do and why?

The messages we show users in the merge request widgets have NEVER been parallel. They're also super wordy. Let's start by changing the wording over to present tense. It gets rid of the 'will be' construction.

• Message 1:
  • OLD: The source branch will be deleted
  • NEW: Deletes the source branch
• Message 2:
  • OLD: The source branch will not be deleted
  • NEW: Does not delete the source branch
• Message 3:
  • OLD: The changes will be merged into [x]
  • NEW: Merges changes into [x]
• Message 4:
  • OLD: %{commitCount} will be added to %{targetBranch}.
  • NEW: Adds %{commitCount} to %{targetBranch}.
• Message 5, %{squashedCommits} - also used in message 6
  • OLD: (commits will be squashed)
  • NEW: (squashes %d commit) and (squashes %d commits)
• Message 6:
  • OLD: %{commitCount} and %{mergeCommitCount} will be added to %{targetBranch}%{squashedCommits}.
  • NEW: Adds %{commitCount} and %{mergeCommitCount} to %{targetBranch}%{squashedCommits}.

Closes #343780 (closed)

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