Update references from master branch to main

[vojtechsokol]

Focus was on making the CI and GitHub actions work after the default
branch was switched from master to main.

See: OAMG-4907

[github-actions]

Thank you for contributing to the Leapp project!

Please note that every PR needs to comply with the Leapp Guidelines and must pass all tests in order to be mergeable.
If you want to request a review or rebuild a package in copr, you can use following commands as a comment:

• review please @oamg/developers to notify leapp developers of the review request
• /packit copr-build to submit a public copr build using packit

Packit will automatically schedule regression tests for this PR's build and latest upstream leapp build.
However, here are additional useful commands for packit:

• /packit test to re-run manually the default tests
• /packit retest-failed to re-run failed tests manually
• /packit test oamg/leapp#42 to run tests with leapp builds for the leapp PR#42 (default is latest upstream - master - build)

Note that first time contributors cannot run tests automatically - they need to be started by a reviewer.

It is possible to schedule specific on-demand tests as well. Currently 2 test sets are supported, beaker-minimal and kernel-rt, both can be used to be run on all upgrade paths or just a couple of specific ones.
To launch on-demand tests with packit:

• /packit test --labels kernel-rt to schedule kernel-rt tests set for all upgrade paths
• /packit test --labels beaker-minimal-8.10to9.4,kernel-rt-8.10to9.4 to schedule kernel-rt and beaker-minimal test sets for 8.10->9.4 upgrade path

See other labels for particular jobs defined in the .packit.yaml file.

Please open ticket in case you experience technical problem with the CI. (RH internal only)

Note: In case there are problems with tests not being triggered automatically on new PR/commit or pending for a long time, please contact leapp-infra.

[matejmatuska]

LGTM, all actions passed. However the commit message could mention it's about Github actions (or CI).