Adding PHP 8.1 testing for 4.8.x

[Yurunsoft]

既然 swoole-cli 支持 PHP 8.1 + 4.8.x，应该把 PHP 8.1 加入测试

[codecov[bot]]

Codecov Report

Merging #4745 (148e1f6) into 4.8.x (43c13c6) will increase coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##            4.8.x    #4745      +/-   ##
==========================================
+ Coverage   52.62%   52.65%   +0.02%     
==========================================
  Files          73       73              
  Lines       14939    14940       +1     
==========================================
+ Hits         7861     7866       +5     
+ Misses       7078     7074       -4     

Impacted Files	Coverage Δ
src/os/async_thread.cc	71.67% <0.00%> (-0.43%)	:arrow_down:
src/server/process.cc	49.69% <0.00%> (-0.31%)	:arrow_down:
src/coroutine/hook.cc	66.78% <0.00%> (ø)
src/core/channel.cc	71.84% <0.00%> (+0.97%)	:arrow_up:
src/protocol/base.cc	64.45% <0.00%> (+1.80%)	:arrow_up:
src/os/msg_queue.cc	61.01% <0.00%> (+3.38%)	:arrow_up:

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