Drop compatibility with PHP 5.6, 7.0 and 7.1

[enricobattocchi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Drops compatibility with PHP 5.6, 7.0 and 7.1.

Relevant technical choices:

• Note that Yoast SEO is not required to run Yoast Duplicate Post, the latter is not an addon.
• Also note that composer.lock is not committed to the repo by design.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Verify the GH actions pass
• install on a PHP 5.6 environment
• visit the Plugins page
• see that you get the same message and you can't activate the plugin, as it happens here for Free and Premium:
  
• the same should happen with 7.0 and 7.1

note: the above may be tricky to test. I can see 5.6 as a choice in Local by Flywheel, but I know it's not the case for others (and I can see 7.0 an 7.1 are not available - neither 7.2 actually)
I tried on Instawp, where you have the full choice of PHP versions, but when uploading the artifact I got:

which is also a nice thing to check, actually.

So a way could be:

• start a 7.2 site on instawp
• upload the artifact but don't activate it
• switch to 5.6
• check that you get the same message above and you can't activate the plugin
• switch to 7.0 an 7.1
• the same should happen

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Block/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label and noted the work hours.

Fixes Yoast/wordpress-seo#19801

[jrfnl]

Reviewed.

• I've double-checked that all references to PHP 5.6 have been updated/removed ✅
• Made the necessary selective updates to the composer.json and composer.lock file.
• Removed the config-platform setting from composer.json (see the commit message).

I've also verified that the tests currently pass on the absolute minimum PHP version by running the unit tests locally against PHP 7.2.5.

The caveat is, of course, that test coverage is low, so this is no guarantee, only a tentative indication.