Fix AppStream metadata in QGIS 3.34.

[sebastic]

Same changes as #59769, but for the 3.34 branch.

Fixes: #59220

[m-kuhn]

Should this also be merged?

3.34 is almost EOL, at this stage only patches that fix serious issues like crashes are permitted. OTOH this seems to be very low risk.
@nyalldawson thoughts?

[github-actions]

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[github-actions]

The QGIS project highly values your contribution and would love to see this work merged! Unfortunately this PR has not had any activity in the last 14 days and is being automatically marked as "stale". If you think this pull request should be merged, please check

• that all unit tests are passing

• that all comments by reviewers have been addressed

• that there is enough information for reviewers, in particular

  • link to any issues which this pull request fixes

  • add a description of workflows which this pull request fixes

  • add screenshots if applicable

• that you have written unit tests where possible
  In case you should have any uncertainty, please leave a comment and we will be happy to help you proceed with this pull request.
  If there is no further activity on this pull request, it will be closed in a week.