include build info in Prometheus metrics (#22819)

Related to: https://github.com/go-gitea/gitea/issues/18061

This PR adds build info to the Prometheus metrics. This includes:
- goarch: https://pkg.go.dev/runtime#GOARCH
- goos: https://pkg.go.dev/runtime#pkg-constants
- goversion: https://pkg.go.dev/runtime#Version
- gitea version: just exposes the existing
code.gitea.io/gitea/modules/setting.AppVer

It's a similar approach to what some other Golang projects are doing,
e.g. Prometheus:
https://github.com/prometheus/common/blob/main/version/info.go

example /metrics response from Prometheus:
```
# HELP prometheus_build_info A metric with a constant '1' value labeled by version, revision, branch, goversion from which prometheus was built, and the goos and goarch for the build.
# TYPE prometheus_build_info gauge
prometheus_build_info{branch="HEAD",goarch="amd64",goos="linux",goversion="go1.19.4",revision="c0d8a56c69014279464c0e15d8bfb0e153af0dab",version="2.41.0"} 1
```

/metrics response from gitea with this PR:
```
# HELP gitea_build_info Build information
# TYPE gitea_build_info gauge
gitea_build_info{goarch="amd64",goos="linux",goversion="go1.20",version="2c6cc0b8c"} 1
```

Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

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Signed-off-by: Michal Wasilewski <mwasilewski@gmx.com>

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