refactor: clarify GOOS detection (#39620)

Introduce `consts.IsWindows`, now it's clearer to see how Windows build works

modules/git/git.go

@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 
 	"gitea.dev/modules/cache"
@@ -126,16 +125,8 @@ func checkGitVersionCompatibility(gitVer *version.Version) error {
 func ensureGitVersion() error {
 	if !DefaultFeatures().CheckVersionAtLeast(RequiredVersion) {
 		moreHint := "get git: https://git-scm.com/downloads"
-		if runtime.GOOS == "linux" {
-			// there are a lot of CentOS/RHEL users using old git, so we add a special hint for them
-			if _, err := os.Stat("/etc/redhat-release"); err == nil {
-				// ius.io is the recommended official(git-scm.com) method to install git
-				moreHint = "get git: https://git-scm.com/downloads/linux and https://ius.io"
-			}
-		}
 		return fmt.Errorf("installed git version %q is not supported, Gitea requires git version >= %q, %s", DefaultFeatures().gitVersion.Original(), RequiredVersion, moreHint)
 	}
-
 	if err := checkGitVersionCompatibility(DefaultFeatures().gitVersion); err != nil {
 		return fmt.Errorf("installed git version %s has a known compatibility issue with Gitea: %w, please upgrade (or downgrade) git", DefaultFeatures().gitVersion.String(), err)
 	}