Make GtS reported version SemVer-compatible

[VyrCossont]

Description

This pull request makes GtS's version string compatible with SemVer by concatenating the VCS metadata with the version using a plus sign (SemVer's build tag separator) instead of a space. It relies on main.Version being set to a SemVer-compatible version, which should be the case for release and most development builds.

Fixes #1953

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 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.

[tsmethurst]

One character change, nice! Looks fine to me, and I can't think of any negatives for doing things this way. @NyaaaWhatsUpDoc any thots?