Change: Allow only manual releases from main

[Thijss]

Changes proposed in this PR include:

Change CI/CD so that only manual releases from the main branch are allowed

Changes related to manual release flow:

• build-test-release now only runs on a push to main or a manual trigger.
  • within build-test-release, the github-release job will now only run on a manual trigger from the main branch
  • within build-test-release, the publish job will now only run if github-release job has run (I moved the code too)

Other changes:

• regular PRs will not test on all operating systems anymore, only on Linux. Only on a push to main or on a manual trigger of the CI flow will all operating systems be tested
• I've removed most merge_group logic, as we don't use it

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud