[feat] Remove support for Python < 3.9 and support for the legacy environment modules version 3

[vkarak]

This PR removes support for all Python versions lower than 3.9 and it also removes support of the legacy environment modules. The following changes are introduced in detail

1. Remove all Python < 3.9 workarounds and specific dependencies
2. Remove Tmod3 backends and merge TmodImpl and Tmod4Impl
3. The tmod4 name is deprecated in favor of envmod which corresponds to the modern environment modules.
4. Unit tests for old Python versions and for unsupported modules systems are removed.
5. Updated the containers for Lmod and Envmod to use their latest version on Ubuntu 24.04.
6. Fixed Lmod collection unit tests as since Lmod 9 the location of module collections has changed.
7. Removed stale ci configuration files and cleaned up the existing ones.
8. Bump the Spack and EasyBuild versions used for unit tests and in the howto.
9. Build the zlib instead of bzip2 in the Spack/EasyBuild examples, as EasyBuild has introduced a dependency on binutils which would cause the increased build times not suitable for CI. The howto examples are updated accordingly.
10. osext.copytree is deprecated as it is no more necessary.
11. Documentation is updated accordingly.

Closes #3583.
Closes #3269.

[codecov]

Codecov Report

❌ Patch coverage is 64.81481% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.38%. Comparing base (07ff6b3) to head (f99e544).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
reframe/core/modules.py	56.81%	19 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3596      +/-   ##
===========================================
+ Coverage    91.19%   91.38%   +0.19%     
===========================================
  Files           62       62              
  Lines        13622    13484     -138     
===========================================
- Hits         12423    12323     -100     
+ Misses        1199     1161      -38     

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[vkarak]

I think there is something wrong with pytest-cov as it doesn't show as covered the imports and the function definitions 👀

[ekouts]

lgtm