Add *.lock.yml to prettierignore and update Pwsh Stable to v7.6.

[liamjpeters]

PR Summary

Prettier objects to many things (mainly double quotes) in .github/workflows/issue-triage.lock.yml.

CI has been failing since #5409.

As the file is auto-generated, telling Prettier to ignore it and any other generated lock files seems a way forward.

Edit:
Also updated the stable version test from accepting v.7.5 to v7.6; v7.6 is now the advertised stable version. I've left the LTS at v7.4 as that hasn't updated yet.

Gets CI back to passing.

PR Checklist

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[liamjpeters]

Build progresses past linting now:

Fails in the PowerShell update tests as the stable build info URL now reports v7.6.0 🎉🎈.

vscode-powershell/test/features/UpdatePowerShell.test.ts

Line 133 in f7b9d20

assert(tag?.startsWith("v7.5") || tag?.startsWith("v7.4"));

[JustinGrote]

Thanks for your submission! We do need to update tests for 7.6.0 but that's separate. LGTM, CC @andyleejordan