dev: Add ability to switch seamlessly between PHP7 and PHP8

[ankitpokhrel]

This PR:

• Removes PECL dependency by compiling apcu and xdebug manually. PECL (along with PEAR) is deprecated since PHP7.4 and might not be available in the future.
• Removes unnecessary dependencies.
• Adds a new make target clean.

This change will now allow us to switch between PHP7 and PHP8 just by changing the FROM directive in the base docker image. For eg: php:8.0.0RC3-fpm-alpine.

[codecov]

Codecov Report

Merging #313 (71eb811) into master (fa24517) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #313   +/-   ##
=========================================
  Coverage     99.68%   99.68%           
  Complexity      351      351           
=========================================
  Files            20       20           
  Lines           940      940           
=========================================
  Hits            937      937           
  Misses            3        3           

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