Warn users when attempting to cherry pick with old key

[jesseduffield]

I wrote this feature and even I sometimes use the wrong key so I want to make this very clear to users so that there's no confusion.

We can get rid of this once users have had time to adjust

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[jesseduffield]

Adding 'ignore for release' label because it's not really something that we need to call out in the release notes

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ 0.00% (target: -2.00%)	✅ 52.63%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (cf5d4d4)	47593	39684	83.38%
Head commit (e1aa68a)	47612 (+19)	39698 (+14)	83.38% (0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3271)	19	10	52.63%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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_{You may notice some variations in coverage metrics with the latest Coverage engine update. For more details, visit the documentation}