Drop support for symfony 4.x

[pableu]

As mentioned by @PhilinTv in #1992.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.28%. Comparing base (f4889ff) to head (e75c167).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1996 +/- ## ========================================= Coverage 89.28% 89.28% Complexity 8058 8058 ========================================= Files 232 232 Lines 24534 24534 ========================================= Hits 21905 21905 Misses 2629 2629 ``` | [Flag](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | Coverage Δ | | |---|---|---| | [5-max](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `89.28% <ø> (ø)` | | | [7.4](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `89.28% <ø> (ø)` | | | [agnostic](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `67.44% <ø> (ø)` | | | [mysql](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `69.83% <ø> (ø)` | | | [pgsql](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `69.87% <ø> (ø)` | | | [sqlite](https://app.codecov.io/gh/propelorm/Propel2/pull/1996/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm) | `67.81% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=propelorm#carryforward-flags-in-the-pull-request-comment) to find out more.

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[PhilinTv]

Finally! 😄 @pableu 👏