Remove the upstream PR welcome message

[prilr]

The message references the upstream oamg leapp project, which could look rather confusing people attempting to contribute to the fork.
We might add a different message in the future, but let's remove the previous one for now.

[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. If you need a different version of leapp from PR#42, use /packit test oamg/leapp#42

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-8.9to9.3,kernel-rt-8.9to9.3 to schedule kernel-rt and beaker-minimal test sets for 8.9->9.3 upgrade path

[Deprecated] To launch on-demand regression testing public members of oamg organization can leave the following comment:

• /rerun to schedule basic regression tests using this pr build and latest upstream leapp build as artifacts
• /rerun 42 to schedule basic regression tests using this pr build and leapp*PR42* as artifacts
• /rerun-sst to schedule sst tests using this pr build and latest upstream leapp build as artifacts
• /rerun-sst 42 to schedule sst tests using this pr build and leapp*PR42* as artifacts

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.