chore: added php 7.3-7.4 compatibility checker

[arzola]

This PR adds an extra check to ensure our codebase doesn't have any deprecated or non compatible function within our supported PHP versions

https://github.com/pressbooks/private/issues/815

[private-packagist]

composer.lock

Package changes

Package	Operation	From	To	Changes
phpcompatibility/php-compatibility	add	-	9.3.5	view code
squizlabs/php_codesniffer	add	-	3.5.8	view code

Dev Package changes

Package	Operation	From	To	Changes
phpcompatibility/php-compatibility	remove	9.3.5	-	-
squizlabs/php_codesniffer	remove	3.5.8	-	-

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[codecov]

Codecov Report

Merging #132 (8d887fd) into dev (c268f0f) will not change coverage.
The diff coverage is n/a.

❗ Current head 8d887fd differs from pull request most recent head 53811b2. Consider uploading reports for the commit 53811b2 to get more accurate results

@@            Coverage Diff            @@
##                dev     #132   +/-   ##
=========================================
  Coverage     79.09%   79.09%           
  Complexity      327      327           
=========================================
  Files             9        9           
  Lines          1172     1172           
=========================================
  Hits            927      927           
  Misses          245      245           

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