Improve serverless/operator-verify.yaml files

[MichalKalke]

Description:

At the beginning of prototyping the CI/CD system, we decided to locate all jobs related to the operator and serverless component in the serverless-verify.yaml and the operator-verify.yaml files. This solution brings a lot of disadvantages related to this kind of division:

• push and pull_request jobs are in the same file and because of this we need to add ifs in every job to determine on which event every job should be run on
• both files contain really similar jobs test kinds - for example, both files contain lint, integration tests, unit tests, and upgrade tests that are almost the same and have the same pre-config
• the GitHub description under the PR or commit (after merging PR) show many skipped jobs

I think we have a few options to change this hierarchy.

1. Split these files into pre/post files that would contain all event-related jobs
2. Split these files into kind-related files like integration-tests.yaml, lint.yaml, and so on
3. Option 1+2 - for example pre-integration-tests.yaml, post-integration-tests.yaml and so on
4. Split these files into smaller files for every job to contain every job separately