Expanding and updating CI

[JordiManyer]

Changes to CI:

• Updated actions to newer versions
• Consolidated x64 and x86 tests in a single file
• Moved documentation building to a new file
• Expanded tests to windows and macos
• New Downgrade workflow: Checks if lower bound compat entries are actually supported

Changes in Gridap

These are changes required by the new CI workflows:

• Cleaned up compats so that the lower-bound compat configuration is actually working.
  • The most radical change comes from bumping up compat for Julia to +1.6. This leaves us with 2 LTSs supported (1.6, 1.10) which should be more than enough.
• I removed the manual cleanup we do within our tests by calling rm(tmpdir). This is messing with windows filesystem, and is no longer needed after Julia 1.3 since temp directories get cleaned up by default.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.88%. Comparing base (b31f36d) to head (2477b6c).
Report is 29 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
+ Coverage   88.74%   88.88%   +0.14%     
==========================================
  Files         195      195              
  Lines       23567    23570       +3     
==========================================
+ Hits        20914    20951      +37     
+ Misses       2653     2619      -34     

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