Normalize MarkDeprecated() messages

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 of the existing 14 messages already follow this style; this MR brings the remaining six in line.

Changes (6 files):

• mr/checkout/mr_checkout.go — Now enabled by default → now enabled by default.
• mr/create/mr_create.go — Use --repo instead. → use --repo instead.
• ci/get/get.go — Deprecated. Use 'output' instead. → use --output instead. (drops the redundant "Deprecated." prefix since cobra already says "has been deprecated,")
• ci/list/list.go — use --order instead → use --order instead. (adds period)
• release/list/release_list.go — Use \glab release view ` instead.` → lowercases first letter
• issuable/list/issuable_list.go — use --assignee=@me → use --assignee=@me.

How to verify

make lint           # 0 issues
make gen-docs       # no changes — these messages don't appear in generated flag help
glab mr checkout --track 1   # see the new deprecation warning

Author's checklist

• make lint passes with 0 issues.
• All 14 MarkDeprecated() calls now follow the same style.
• No semantic change; pure presentation.