Remove Symfony <4.4 support

[fbourigault]

Fix partially #1629

[codecov-commenter]

Codecov Report

Merging #1816 (fb20f4c) into master (d9817c1) will increase coverage by 0.28%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1816      +/-   ##
============================================
+ Coverage     87.83%   88.12%   +0.28%     
+ Complexity     7836     7774      -62     
============================================
  Files           281      279       -2     
  Lines         22748    22637     -111     
============================================
- Hits          19981    19948      -33     
+ Misses         2767     2689      -78     

Flag	Coverage Δ
5-max	88.12% <100.00%> (+0.28%)	:arrow_up:
7.4	88.12% <100.00%> (+0.28%)	:arrow_up:
agnostic	68.88% <50.00%> (+0.33%)	:arrow_up:
mysql	70.73% <100.00%> (+0.19%)	:arrow_up:
pgsql	70.61% <100.00%> (+0.19%)	:arrow_up:
sqlite	68.64% <100.00%> (+0.18%)	:arrow_up:

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Impacted Files	Coverage Δ
src/Propel/Common/Config/PropelConfiguration.php	100.00% <100.00%> (+0.32%)	:arrow_up:
src/Propel/Generator/Command/InitCommand.php	68.53% <100.00%> (+0.58%)	:arrow_up:

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[dereuromark]

The conflicts are why I wanted this to be a clear order of PRs (or the same one) :) To keep work at a minimum.