[docs] Update how to enable http2 on nginx

[daenney]

Description

Since 1.25.1 the newer 'http2 on;' syntax should be used. The previous syntax still works, but throws warnings when testing the configuration with 'nginx -t'.

This also updates the certbot template to match what's currently generated. It removes ipv6only=on as that's the default on a listen directive binding on a wildcard IPv6 address.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.