Change references to default branch: master -> main

[juhoinkinen]

Changes in:

• GH Actions
  • CI/CD workflow
  • CodeQL workflow
• README.md
• CONTRIBUTING.md
• docs/index.rst (for ReadTheDocs content)

Related to #679.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (eb437a8) 99.57% compared to head (6037cff) 99.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #680 +/- ## ======================================= Coverage 99.57% 99.57% ======================================= Files 87 87 Lines 6157 6157 ======================================= Hits 6131 6131 Misses 26 26 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NatLibFi). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=NatLibFi)

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
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