Fix required version numbers

[enricobattocchi]

Summary

This PR can be summarized in the following changelog entry:

• Sets the minimum required Yoast SEO version to 20.8.
• Sets the minimum required ACF version to 6.0.0.

Relevant technical choices:

*

Test instructions

This PR can be tested by following these steps:

• Install with Yoast 20.8-RC and ACF 6. and see you don't get any notice
• Install with Yoast 20.7 and see you get a notice "Please upgrade the Yoast SEO plugin to the latest version to allow the ACF Content Analysis for Yoast SEO module to work."
• Install with ACF 5.* and see you get a notice "ACF Content Analysis for Yoast SEO requires ACF (free or pro) 6.0.0 or higher to be installed and activated."
• Install without Yoast SEO and see you get a notice "ACF Content Analysis for Yoast SEO requires Yoast SEO to be installed and activated."

Fixes #

[coveralls]

Pull Request Test Coverage Report for Build 4991320433

• 1 of 9 (11.11%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 52.675%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
inc/dependencies/dependency-acf.php	1	4	25.0%
inc/dependencies/dependency-yoast-seo.php	0	5	0.0%
<!--	Total:	1	9	11.11%	-->

Totals
Change from base Build 4926431578:	0.2%
Covered Lines:	128
Relevant Lines:	243

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💛 - Coveralls