Increase minimum supported Chrome and Firefox versions

[kzar]

We agreed to support back to the previous ESR Firefox
release[1] (currently 91) and Chrome releases for the past
year[2] (currently back to 92). We will bump these minimum version
numbers periodically in line with that policy.

1 - https://endoflife.date/firefox
2 - https://chromiumdash.appspot.com/releases?platform=Windows

Reviewer: @ladamski

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications