upgrade to PHP 8.1

[mimmi20]

Fixes #89

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	20		0	0.04s
✅ MARKDOWN	markdownlint	2		0	0.29s
✅ MARKDOWN	markdown-link-check	2		0	2.6s
✅ PHP	php	9		0	0.15s
✅ PHP	phplint	9		0	0.12s
✅ YAML	prettier	3		0	0.57s
✅ YAML	yamllint	3		0	0.12s

See errors details in artifact MegaLinter reports on CI Job page

[codecov]

Codecov Report

Merging #90 (937a580) into master (d0d9963) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #90   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        25        27    +2     
===========================================
  Files              4         4           
  Lines             63        67    +4     
===========================================
+ Hits              63        67    +4     

Flag	Coverage Δ
7.4	?
8	?
8.1	100.00% <100.00%> (ø)
php-7.4	?
php-8.0	?
php-8.1	100.00% <100.00%> (ø)
phpunit	100.00% <100.00%> (ø)
ubuntu-18.04	?
ubuntu-20.04	100.00% <100.00%> (ø)
ubuntu-22.04	100.00% <100.00%> (ø)
xdebug	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/Router/HostNameFactory.php	100.00% <ø> (ø)
src/Router/HostName.php	100.00% <100.00%> (ø)

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