Move admin to includes directory

[thelovekesh]

Summary

Previously #1042

As discussed in #1029 (comment), we discussed moving the top-level PL plugin code to the includes directory. In #1042 we already moved the site-health and server-timing to the includes directory but it seems like the admin directory was not moved to the right place. This PR aims to do that.

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Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[westonruter]

LGTM

[felixarntz]

Great catch, thanks @thelovekesh!