task(settings): Add reg_age_invalid glean metric

[dschom]

Because

• We want a metric recorded when a user enters an invalid age

This pull request

• Creates the reg_age_invalid glean metric
• Fires the event on the cannot_create_account page

Issue that this pull request solves

Closes: FXA-9421

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Other information (Optional)

Test instructions

• Signup for an account
• Enter email
• Enter passwords
• For age enter 10
• Click Continue
• Observe the page saying 'You must meet certain age requirements to create a ⁨Mozilla account⁩.'
• Observe that the reg_invalid_age metric was fired