Update default minimum WP version

[jrfnl]

The minimum version should be three versions behind the latest WP release, so what with 4.9 having come out last week, it should now be 4.6.

@JDGrimes Want to run your deprecated function magic to see if we need to add functions/classes/parameters to the lists ?

[jrfnl]

The build is running into the same fixer conflict issue as #1233. I'll look into that when I find some time and open a separate PR for that if needs be. (This one would need to be rebased afterwards)

[jrfnl]

Rebased. I expect the build will now pass.

[JDGrimes]

@jrfnl I've run the tool, and it looks like there are some newly deprecated functions in 4.9. Here is a diff: feature/update-minimum-wp-version...feature/update-minimum-wp-version-diff

The other differences I think are mostly because of the scanner's limitations (it doesn't detect class methods), but it might be good to double-check on some.

[jrfnl]

@JDGrimes Thanks for that! I've re-verified all the differences.

• The methods are covered by ticket Support checking for methods in addition to functions #731 and should be removed once that tickets gets fixed.
• The function differences are fine. The sniff is correct in this case and WP Core is lagging behind. These have all been previously reported in https://core.trac.wordpress.org/ticket/41121 (still open).
  I have added comments to the sniff to annotate this and save me from having to do the same check again next time ;-)

Other than that, the four newly deprecated functions have of course been added, so this PR should be good to go AFAICS.

[JDGrimes]

Should this be merged as part of 0.15.0, or should it wait?

[jrfnl]

@JDGrimes As 4.9 is out and it's a small change, I'd be ok with it to be merged in for 0.15.0.