Warning link doesn't point to anything anymore

The following warning points to a nonexistent link:
```
[GIN-debug] [WARNING] You trusted all proxies, this is NOT safe. We recommend you to set a value.
Please check https://pkg.go.dev/github.com/gin-gonic/gin#readme-don-t-trust-all-proxies for details.
```
## Environment

- go version: 1.22.2
- gin version (or commit ref): v1.9.1
- operating system: Linux
