phpcs updates

[tijmenbruggeman]

Updates and additions to formatting dependencies. This PR will also help ensure the plug-in stays compatible with php5.6.

Changes

• Update license to GPL-2.0-or-later. License "GPL-2.0+" is a deprecated SPDX license identifier, use "GPL-2.0-or-later" instead
• Added php-compatibility checks. This check will ensure that our plug-in stays compatible with the defined version (in this case php5.6)
• Added php-compatibility-wp as wordpress adds some polyfills. This ensures the compatiblity checker doesn't fall over those additions.
• updated phpcs and wordpress codestyles, ran autoformatter
• manual fixes
  • line endings
  • use gmdate instead of date
  • explicit visibility for methods
  • allow one class per file
  • strict in_array comparisons for types
  • ignore strict comparison check (===) as this would functionally change the workings and I don't want to add functional changes in this PR.