fix: normalize MarkDeprecated() messages to lowercase with trailing period

What does this MR do and why?

Normalizes the 14 MarkDeprecated() calls across the codebase to use a consistent style. Cobra renders these as:

Flag --X has been deprecated, <message>

so the message reads most naturally with a lowercase first letter and a trailing period, treated as a sentence continuation. Eight already follow this style; this MR brings the remaining six in line.

Changes (6 files): mr/checkout, mr/create, ci/get, ci/list, release/list, issuable/list.

(Resubmission of !3191 (closed) — the original fork was deleted by storage cleanup; same content reapplied to a fresh fork.)

make lint passes (0 issues). No generated-docs changes.