Patch phpunit to allow testing in PHP 8.1

[vearutop]

Resolves #130.

PHP 8.1 introduced strict requirement to match function header when implementing an interface, unfortunately this makes a breaking change for older versions of PHP without any meaningful benefit. Fortunately these requirements can be suppressed, which was done in this PR.

This PR adds PHP 8.1 to CI pipeline, downgrades to older phpunit to support PHP 5.4 and patches phpunit to support PHP 8.1.

[github-actions]

Lines Of Code

Language	Files	Lines	Code	Comments	Blanks	Complexity	Bytes
JSON	22	5315 (-1)	5315 (-1)	0	0	0	145.7K (-33B)
Makefile	1	38 (+6)	27 (+4)	0	11 (+2)	7	2.2K (+212B)
Markdown	2	636 (+6)	488 (+4)	0	148 (+2)	0	20K (+173B)
PHP	133	11161 (+18)	8067 (+2)	1495 (+14)	1599 (+2)	559	323.2K (+596B)
Patch	1 (+1)	167 (+167)	149 (+149)	0	18 (+18)	0	5.5K (+5.5K)
YAML	4	147	132	1	14	0	3.7K (+101B)

[codecov]

Codecov Report

Merging #131 (94f1e47) into master (1d7cce2) will increase coverage by 0.10%.
The diff coverage is n/a.

❗ Current head 94f1e47 differs from pull request most recent head a832b98. Consider uploading reports for the commit a832b98 to get more accurate results

@@             Coverage Diff              @@
##             master     #131      +/-   ##
============================================
+ Coverage     92.53%   92.64%   +0.10%     
============================================
  Files            27       27              
  Lines          1688     1686       -2     
============================================
  Hits           1562     1562              
+ Misses          126      124       -2     

Impacted Files	Coverage Δ
src/Constraint/Properties.php	85.71% <ø> (ø)
src/MagicMapTrait.php	83.78% <ø> (ø)
src/Structure/ClassStructureTrait.php	95.83% <ø> (ø)
src/Structure/ObjectItemTrait.php	76.19% <ø> (+2.34%)	⬆️
src/Wrapper.php	59.01% <ø> (ø)

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