1022 wrong compiler version in ci for gcc

[MaxBetzDLR]

Changes and Information

Please briefly list the changes (main added features, changed items, or corrected bugs) made:

• Use gcc/gxx-11 in CI
• Update Readme

Merge Request - Guideline Checklist

Please check our git workflow. Use the draft feature if the Pull Request is not yet ready to review.

Checks by code author

• [x] Every addressed issue is linked (use the "Closes #ISSUE" keyword below)
• [x] New code adheres to coding guidelines
• [x] No large data files have been added (files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• [x] Tests are added for new functionality and a local test run was successful (with and without OpenMP)
• [ ] Appropriate documentation for new functionality has been added (Doxygen in the code and Markdown files if necessary)
• [ ] Proper attention to licenses, especially no new third-party software with conflicting license has been added
• [ ] (For ABM development) Checked benchmark results and ran and posted a local test above from before and after development to ensure performance is monitored.

Checks by code reviewer(s)

• [ ] Corresponding issue(s) is/are linked and addressed
• [ ] Code is clean of development artifacts (no deactivated or commented code lines, no debugging printouts, etc.)
• [ ] Appropriate unit tests have been added, CI passes, code coverage and performance is acceptable (did not decrease)
• [ ] No large data files added in the whole history of commits(files should in sum not exceed 100 KB, avoid PDFs, Word docs, etc.)
• [ ] On merge, add 2-5 lines with the changes (main added features, changed items, or corrected bugs) to the merge-commit-message. This can be taken from the briefly-list-the-changes above (best case) or the separate commit messages (worst case).

Closes #1022

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.23%. Comparing base (bcfa8d3) to head (9e6b4b6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
- Coverage   96.51%   96.23%   -0.29%     
==========================================
  Files         128      128              
  Lines       10538    10852     +314     
==========================================
+ Hits        10171    10443     +272     
- Misses        367      409      +42     

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