Fix not serializable classes for 8.1.0beta1

[remicollet]

From UPGRADING.INTERNALS

 f. Non-serializable classes should be indicated using the
     ZEND_ACC_NOT_SERIALIZABLE (@not-serializable in stubs) rather than the
     zend_class_(un)serialize_deny handlers which are removed.

[codecov]

Codecov Report

Merging #4335 (e52686d) into master (029f174) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4335      +/-   ##
==========================================
+ Coverage   50.62%   50.68%   +0.06%     
==========================================
  Files          72       72              
  Lines       14516    14516              
==========================================
+ Hits         7349     7358       +9     
+ Misses       7167     7158       -9     

Impacted Files	Coverage Δ
src/os/process_pool.cc	57.89% <0.00%> (+0.45%)	⬆️
src/os/async_thread.cc	68.85% <0.00%> (+1.75%)	⬆️
src/protocol/base.cc	63.15% <0.00%> (+1.97%)	⬆️

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