[feat] Introduce ReFrame 4.0

[vkarak]

This PR requests to merge the 4.x branch of ReFrame into master introducing ReFrame 4.0. So far, this PR removes all current deprecations. Development for the 4.0 version will continue from there.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@ad792b0). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #2562   +/-   ##
=========================================
  Coverage          ?   86.29%           
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  Files             ?       59           
  Lines             ?    10897           
  Branches          ?        0           
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  Hits              ?     9404           
  Misses            ?     1493           
  Partials          ?        0           

Impacted Files	Coverage Δ
reframe/core/containers.py	100.00% <ø> (ø)
reframe/core/launchers/__init__.py	100.00% <ø> (ø)
reframe/core/schedulers/__init__.py	98.29% <ø> (ø)
reframe/core/systems.py	91.35% <ø> (ø)
reframe/utility/__init__.py	92.60% <ø> (ø)
reframe/utility/sanity.py	97.79% <ø> (ø)
reframe/__init__.py	81.81% <100.00%> (ø)
reframe/core/decorators.py	66.96% <100.00%> (ø)
reframe/core/pipeline.py	93.92% <100.00%> (ø)
reframe/utility/versioning.py	95.23% <100.00%> (ø)

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[ekouts]

lgtm, I just have one small question