Consistent handling of features added/removed in multiple PHP minors

As per the comments in #545 / #544:

Some functions/parameters/extensions/other features are added/removed in several different PHP versions, like the OCI functions discussed in #545, which were added in PHP 7.0.21/23, 7.1.7/9 and 7.2.0.

This is most often the case when it concerns a PHP extension - in contrast to PHP Core -.

We may want to take a principle decision (always use lowest/highest PHP version where the function first appeared/was deprecated in last) and verify that the logic is applied consistently throughout the PHPCompatibility sniffs.

Oh and add the decision taken to the contributors guidelines so it's documented.

Alternatively, we could try and find a way to check against minors - related #531 -, though in that case, we'd still need a principle decision as - taking the above example - PHP 7.0.25 would contain the functions, but PHP 7.1.3 would not, so what should be reported when a testVersion of 7.0.25- would have been provided ?

Start by reading the comments and examples in #545 and #544, then compare the existing PHPCompatibility sniffs that handle features added or removed across PHP minors. Review #531 for the minor-version approach. Done means an agreed principle is applied consistently, with the decision recorded in the contributors guidelines.