Deprecation of old Firefox versions

[cadeyrn]

With the release of Firefox 80 and Firefox ESR 78.2 in August 2020 Mozilla won't support Firefox ESR 68 any longer, therefore the Firefox 68 support in Enterprise Policy Generator will be deprecated with the release in August 2020.

• bump minimum required Firefox version from 68.0 to 78.0
• change the following code:

  enterprise-policy-generator/src/js/core/configurator.js

  Line 1356 in a127689

  if (parseFloat(policy.first_available.mainstream) > FIREFOX_68 || parseFloat(policy.first_available.esr) > FIREFOX_68) {

• don't forget to add a note to the CHANGELOG about the new requirement and to update the README
• remove versions notes for policies older than Firefox 78 from translation files