www/caddy: Change ACME E-Mail to required field

[Monviech]

In the upcoming Caddy Version 2.8.x, there were changes with ZeroSSL that now demand the ACME E-Mail to be filled out with a valid E-Mail address to create the ACME account.

So the field has been changed to required.

Please note that this change needs the updated general.volt from this pull request to properly display validation errors, since that has been a bug: https://github.com/opnsense/plugins/pull/3957

Reference: https://github.com/opnsense/plugins/issues/3951