Bump Go version to 1.23.6 (#30242)

* Bump Go version to 1.23.6

* Update CodeQL Github action as well

* Use server's Go version for CodeQL action

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>

* Empty commit to trigger CI

* Bump golangci-lint to a version supporting Go 1.23

* Fix golangci-lint warnings

Several rules from gosimple, revive and staticcheck linters were
failing:
- Redefinition of built-in identifiers (max, min, new, recover...)
- Use of printf-like functions with simple strings
- Check for nil slices, when len already takes it into account

---------

Co-authored-by: Antonis Stamatiou <stamatiou.antonis@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/client4.go

@@ -774,7 +774,7 @@ func (c *Client4) DoAPIRequestReader(ctx context.Context, method, url string, da
 		rq.Header.Set(HeaderAuth, c.AuthType+" "+c.AuthToken)
 	}
 
-	if c.HTTPHeader != nil && len(c.HTTPHeader) > 0 {
+	if len(c.HTTPHeader) > 0 {
 		for k, v := range c.HTTPHeader {
 			rq.Header.Set(k, v)
 		}