[feat] Deprecate variables in favour of the new env_vars

[vkarak]

The use of variables is deprecated in both the configuration and the tests. Users should use env_vars instead.

This PR builds on top of #2647.

Closes #1774.

Todos

• Update tutorial examples

[codecov-commenter]

Codecov Report

Base: 86.27% // Head: 86.30% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (30284d7) compared to base (425a14d).
Patch coverage: 90.15% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2648      +/-   ##
==========================================
+ Coverage   86.27%   86.30%   +0.02%     
==========================================
  Files          60       60              
  Lines       10992    11081      +89     
==========================================
+ Hits         9483     9563      +80     
- Misses       1509     1518       +9     

Impacted Files	Coverage Δ
reframe/core/systems.py	89.95% <ø> (ø)
reframe/frontend/cli.py	72.30% <0.00%> (-0.12%)	⬇️
reframe/frontend/runreport.py	94.61% <33.33%> (-0.70%)	⬇️
reframe/frontend/statistics.py	96.06% <83.33%> (+0.06%)	⬆️
reframe/core/environments.py	94.82% <85.71%> (-1.57%)	⬇️
reframe/core/variables.py	95.75% <94.28%> (-0.56%)	⬇️
reframe/core/config.py	91.82% <96.15%> (+0.28%)	⬆️
reframe/core/logging.py	77.12% <100.00%> (+0.11%)	⬆️
reframe/core/meta.py	99.08% <100.00%> (ø)
reframe/core/pipeline.py	93.41% <100.00%> (+<0.01%)	⬆️
... and 2 more

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[giordano]

Out of curiosity, what changed since #1774 (comment)

As discussed, we will not deprecate variables because this would cause a major disruption.

?

[vkarak]

Out of curiosity, what changed since #1774 (comment)

We were quite ambivalent with this change to be honest, but since 4.0 is a major version bump, we eventually thought that it's probably the right moment to introduce such a change. After all, all it requires is a search-and-replace. Deprecating perf_patterns in favour of @performance_functions on the other hand would be more tedious for the users, so atm we choose not to do so and just update the docs.

FYI, though, there is a bug in the master regarding the use of variables (see #2681). We will fix this asap.