Update CI branch triggers

[mfl28]

• Triggers CI only on pull_requests and pushes to master branch.
• Explicitly sets GA runner macOS version to 10.15 (jpackage fails on latest macOS runner).

Closes #2 .

[codecov]

Codecov Report

Merging #3 (f59a256) into master (c968047) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #3   +/-   ##
=========================================
  Coverage     83.01%   83.01%           
  Complexity     1194     1194           
=========================================
  Files            86       86           
  Lines          5429     5429           
  Branches        474      474           
=========================================
  Hits           4507     4507           
  Misses          677      677           
  Partials        245      245           

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information