End of tested support for PHP 7.3

[whikloj]

As Github will be ending support for the ubuntu-18.04 image as of 2023-04-01 and the ubuntu-20.04 only supports PHP 7.4 I will be ending support for PHP 7.3 at that time.

This will include:

• updating the testing matrix to remove PHP 7.3
• updating composer.json start with PHP 7.4
• releasing a new major version

This library may still work with PHP 7.3 after that moment but it won't be a guarantee and you'll have to update the composer.json yourself.

[whikloj]

Support has ended with https://github.com/whikloj/BagItTools/commit/9f72e2e17774b67dd129f82db016ab5b36a0227f 4.2.3 was the last version to offer PHP 7.3 support.