SO-6267: Update handling of validation thread pool size

[apeteri]

• Change the property name from numberOfValidationThreads to workerPoolSize
• Allow setting either property for backwards compatibility (setting both results in undefined behavior)
• Use null as the default value
• Increase allowed maximum pool size to 99
• Compute a reasonable default value on the server side if the value remains unset in Snow Owl's configuration file

[cmark]

❤️

[cmark]

🥓

[nagyo]

LGTM!

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 48.57%. Comparing base (c0d0904) to head (bb2a754).
Report is 286 commits behind head on 9.x.

Files with missing lines	Patch %	Lines
...e/internal/validation/ValidationConfiguration.java	25.00%	3 Missing ⚠️
...owl/core/internal/validation/ValidationPlugin.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                9.x    #1320      +/-   ##
============================================
- Coverage     48.57%   48.57%   -0.01%     
+ Complexity    14074    14071       -3     
============================================
  Files          1949     1949              
  Lines         95379    95380       +1     
  Branches      11024    11025       +1     
============================================
- Hits          46334    46329       -5     
- Misses        46016    46018       +2     
- Partials       3029     3033       +4     

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