Write project file as a .qgz instead of .qgs file

[MattF-NSIDC]

Description

Closes #777

Release rc2 after this and #783 ?

Checklist

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• The PR description links to issues that it resolves with closes #{issue_number}
• Config lockfile updated (inv config.export > qgreenland/config/cfg-lock.json)
• Environment lockfile updated if needed (conda-lock)
• Version bumped if needed (bumpversion (major|minor|patch|prerelease|build))
• CHANGELOG.md updated (for user-facing changes)
• Documentation updated if needed
• New unit tests if needed

[MattF-NSIDC]

Just testing locally (--include=background) before marking ready for review.

[trey-stafford]

Testing on my end now...

[MattF-NSIDC]

Testing passed by Matt, Trey, Twila