Bump minimum required PHP version to 7.0 and minimum required WP version to 6.3

[mukeshpanchal27]

Summary

Fixes #606

• Bump main plugin minimum required WP version to 6.3
• Bump standalone plugin minimum required WP version to 6.3.
• Bump main plugin minimum required PHP version to 7.0.
• Bump standalone plugin minimum required PHP version to 7.0.
• Update/remove legacy code.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[mukeshpanchal27]

@felixarntz, should we include the update of the standalone plugin version as part of this PR, or would it be better to handle it in a separate PR?

[felixarntz]

Thanks @mukeshpanchal27, LGTM!

Happy to merge this despite the one outstanding point mentioned by @swissspidy. Let's open a separate PR for that, should be a quick one.

[felixarntz]

@mukeshpanchal27

@felixarntz, should we include the update of the standalone plugin version as part of this PR, or would it be better to handle it in a separate PR?

Just saw this comment. How about we merge this PR as is, and then open another PR that does the following:

• remove Fetchpriority module/plugin completely
• bump patch version of WebP Uploads plugin

See the new #853.

Note that Dominant Color Images doesn't need any version bump because it's not released yet anyway.

cc @swissspidy

[spacedmonkey]

Composer install is not failing.

[aristath]

LGTM 👍
Pre-approving, pending the fix for a failing composer issue