Remove Compatibility for PHP 7.3 and Below

[bshaffer]

Remove compatibility for PHP 7.3 and below by making the following change:

Requirements

• Remove tests for PHP 7.3 and below
• Update require.php in composer.json
• Remove support older versions of dependent libraries (Guzzle 5, monolog/monolog:1, ramsey/uuid, google/common-protos)
• Remove phpunit 4/5 and roave phpunit-polyfill

Enhancements

• Use null-coalescing operator (??) instead of isset and ternary operators.

  // PHP 5.6
  $var = isset($array['key']) ? $array['key'] : 'default';
  // PHP 7.4
  $var = $array['key'] ?? 'default';

• Add return typehints
• Add primitive type hints
• Add typed properties
• use PHP Arrow Functions
• replace calls to call_user_func_array with the spread operator

[bshaffer]

Closing this as all required tasks are complete. Opened #6213 so we can track the enhancements