Add dependency on symfony/polyfill-php80 to define ValueError

[osma]

Related to #1170 and especially this comment by @hekl

Skosmos started throwing ValueError exceptions in some situations since PR #1127. However, ValueError was only added in PHP 8, and we are still (mostly) using PHP 7. ValueError still usually worked, because symfony/polyfill-php80 declared it, but it was only available when developmend dependencies were installed.

This PR adds an explicit dependency on symfony/polyfill-php80 also for non-development installs, ensuring that ValueError is always available.

[codecov]

Codecov Report

Merging #1200 (e0374ff) into master (7a570ff) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1200   +/-   ##
=========================================
  Coverage     68.71%   68.71%           
  Complexity     1598     1598           
=========================================
  Files            32       32           
  Lines          3929     3929           
=========================================
  Hits           2700     2700           
  Misses         1229     1229           

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[kouralex]

Works as intended (switches the error from "missing class ValueError" to "Uncaught ValueError").

Can be merged.