Update Symfony to 5.1

[marmichalski]

Related to #206, #239

[marmichalski]

Seems there is an issue with 5.1. Will resolve it when I'm back at keyboard 😎

[akondas]

🤔 recently a lot of this type of errors after updates

[codecov]

Codecov Report

Merging #248 into master will decrease coverage by 0.04%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master     #248      +/-   ##
============================================
- Coverage     99.68%   99.63%   -0.05%     
- Complexity     1513     1514       +1     
============================================
  Files           250      250              
  Lines          4404     4404              
============================================
- Hits           4390     4388       -2     
- Misses           14       16       +2     

Impacted Files	Coverage Δ	Complexity Δ
src/Query/Admin/Proxy/Model/Package.php	80.00% <0.00%> (-20.00%)	4.00 <1.00> (+1.00)	⬇️
src/Command/ProxySyncMetadataCommand.php	100.00% <100.00%> (ø)	4.00 <1.00> (ø)
src/Security/GitLabAuthenticator.php	100.00% <100.00%> (ø)	7.00 <1.00> (ø)

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[karniv00l]

I couldn't push directly so I've made PR out if this #250

[akondas]

Thanks, will be merged in #250