cancel sync-strings on non-beta versions

[escapewindow]

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-commenter]

Codecov Report

Merging #19776 (26df1e8) into master (6dbe5ac) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #19776   +/-   ##
=========================================
  Coverage     35.77%   35.77%           
  Complexity     1648     1648           
=========================================
  Files           545      545           
  Lines         21896    21896           
  Branches       3263     3263           
=========================================
  Hits           7833     7833           
  Misses        13160    13160           
  Partials        903      903           

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