Consolidate to single constant for minimum PHP version

[agh1]

See civicrm/civicrm-core#16753

[civibot]

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[agh1]

The regex was looking for x.y.z, and source data is x.y only. I updated it to ignore the point release unless it's non-zero.

[totten]

Looks good. 👍 Change makes sense. The relevant test passes. I did an r-run on php70 and php71 with drush (drush ev 'civicrm_initialize(); echo "hi\n";'), and (on php70) it complained correctly.

Jenkins reported test failures, but they're a separate/pre-existing issue with pending patches in civicrm-core.git and backdrop.git.