Disallow version_compare() $operator abbreviations

`version_compare()` does a sloppy check for the `$operators` argument
which allows arbitrary abbreviations of the supported operators t

Disallow version_compare() $operator abbreviations

`version_compare()` does a sloppy check for the `$operators` argument
which allows arbitrary abbreviations of the supported operators to be
accepted. This is both undocumented and unexpected, and could lead to
subtle BC breaks, if the order of the comparisions will be changed.
Therefore we change to strict comparisons.

Closes GH-6510.

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