[chore] Web Push: Use server URL for VAPID sub claim

[VyrCossont]

Description

webpush-go now supports sending https:// as well as mailto: URLs for VAPID sub claims, so we can revert 464d920 now and save fetching the instance contact email or making up a dummy email if there wasn't one configured.

Tested on Safari, which was the only browser that cared about whether VAPID sub claims were even correctly formed.

Checklist

• 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.