Improvement: Automate the creation of a version tag on a monthly basis

[Davknapp]

Closes #2032

With 4.0.0 we want to switch to a rolling release. This wf automizes it.
See https://github.com/DLR-AMR/t8code/releases/tag/v4.0.0-2025.12.09 as an example.
Delete that tag befor merging into main.

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• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
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• The author made sure that, as a reviewer, he/she would check all boxes below.

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General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
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If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

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• The author added a BSD statement to doc/ (or already has one).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.75%. Comparing base (6fd67dc) to head (527d311).
⚠️ Report is 35 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2031   +/-   ##
=======================================
  Coverage   76.75%   76.75%           
=======================================
  Files         109      109           
  Lines       18750    18750           
=======================================
  Hits        14392    14392           
  Misses       4358     4358           

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[lenaploetzke]

Nice and seems to work :)

[lenaploetzke]

Ah and the tarball is missing as you pointed out.

[Davknapp]

Ah and the tarball is missing as you pointed out.

I don't think that this is actually a problem. Github automatically adds a tarball to the tag. We should try to build t8code from that. Than we don't have to create it manually.

[Davknapp]

Ah and the tarball is missing as you pointed out.

I don't think that this is actually a problem. Github automatically adds a tarball to the tag. We should try to build t8code from that. Than we don't have to create it manually.

We can not do that, because the version is not properly set in the tarball.

[lenaploetzke]

Thank you for your changes!