Guard the firebase-tools version-check message against drifting from the enforced minimum #3703

Background

src/schematics/firebaseTools.ts enforces a minimum firebase-tools version and prints an error when it isn't met. The enforced version and the message are two separate string literals:

if (semverCompare(version, '14.0.0') === -1) {
    console.error('firebase-tools version 14.0.0+ is required, please upgrade and run again');

These drifted once: when the check was raised from 13 to 14 (v20 release prep, c1c6af9), the message kept saying "13.0.0+" until it was corrected in #3662.

Proposal

Derive both from a single minFirebaseToolsVersion constant so they can't disagree again. No behavior change.