Add PHP 8.1 support, drop PHP 7.4

[Slamdunk]

• Prepare for PHP 8.1
• PHP 8.1 compatibility

[codecov]

Codecov Report

Merging #527 (a24c9f3) into master (5f1d8ed) will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #527      +/-   ##
============================================
- Coverage     95.64%   95.42%   -0.22%     
- Complexity      355      356       +1     
============================================
  Files            45       45              
  Lines           849      853       +4     
============================================
+ Hits            812      814       +2     
- Misses           37       39       +2     

Impacted Files	Coverage Δ
src/ImapResource.php	100.00% <100.00%> (ø)
src/Message/Transcoder.php	88.57% <0.00%> (-2.86%)	⬇️
src/Mailbox.php	96.96% <0.00%> (-0.95%)	⬇️

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