Drop support for Twig 1 and Twig 2

[MauricioFauth]

If PHP 7.2+ is required, there's no reason to support old Twig versions anymore.

I'm thinking about releasing this with the version 4.2.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5aea011) to head (ab2ff00).
⚠️ Report is 34 commits behind head on 4.1.x.

Additional details and impacted files

@@             Coverage Diff             @@
##               4.1.x       #30   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        79        89   +10     
===========================================
  Files              3         4    +1     
  Lines            208       220   +12     
===========================================
+ Hits             208       220   +12     

Flag	Coverage Δ
unit-7.2	96.93% <ø> (?)
unit-7.2-ubuntu-latest	?
unit-7.3	97.22% <ø> (?)
unit-7.3-ubuntu-latest	?
unit-7.4-ubuntu-latest	?
unit-8.0-ubuntu-latest	?
unit-8.1-ubuntu-latest	?
unit-8.2	97.68% <ø> (?)
unit-8.2-ubuntu-latest	?
unit-8.3-ubuntu-latest	?
unit-8.4	97.68% <ø> (?)
unit-8.4-ubuntu-latest	?
unit-8.5	97.68% <ø> (?)

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