Bump MINIMUM_PHP for 6.3

[johnbillion]

Josepha has confirmed that we'll go ahead and bump the minimum supported version of PHP to 7.0.0 for WordPress 6.3.

The MINIMUM_PHP constant for ServeHappy needs to be bumped accordingly. Is there anything else that needs to be changed so that the updates API functions as expected?

Ref: ​https://core.trac.wordpress.org/ticket/57345

[SergeyBiryukov]

In 12714:

API: ServeHappy: Bump the minimum required PHP version to 7.0, to match the version bump in WordPress 6.3.

Follow-up to [8742], ​[WP56141].

Props johnbillion.
Fixes #7126. See ​#WP57345.

[dd32]

In r20612-dotorg:

API: Core Version Check: Specify WP 6.3+ as PHP7 minimum.

See https://meta.trac.wordpress.org/ticket/7126

The version-check response now shows the correct PHP minimum version after the above.

In r20613-dotorg:

API: Core Version Check: Offer WordPress 6.2.xx to the remaining PHP 5.6.20-5.6.9999
     users upon WordPress 6.3's release.

WordPress 5.1 is offered to version of PHP older than 5.6.20.

See https://meta.trac.wordpress.org/ticket/7126

That doesn't currently change the API response, but upon the first 6.3.x release that triggers a 6.2.x release being made, the remaining WP 6.2 sites will see both 6.3.x & 6.2.x, with only the latter being an update destination.
(NOTE: This behaviour is only present for STABLE releases, if you're running a development version, a beta version, etc, you do NOT get an older-version offering, this is specifically for end-users)