Reuse title information as summary

[justusbunsi]

These changes remove the hard-coded summary that links to the repository issue page.
I had a peak on how the xunit plugin provides such status check details.
They split test result details from overall summary.

Unfortunately, github-checks-plugin and gitea-checks-plugin seem to use different parts of the provided status check information. That's why every test report on any Jenkins plugin already has the passed: xyz as status check details.
Gitea seems to receive the static repo issue "fallback" instead.
So, splitting the currently provided information like xunit does it would probably be a somewhat breaking change.
That's why I simply reused the text provided as "Title".

Resolves #674

Testing done

• Unit test execution via mvn clean verify
• Plugin usage in Jenkins, screenshots below for different behaviors

Single failing test:

Multiple failing tests:

All passing tests:

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[justusbunsi]

@timja Text is now calculated once.