Refactor setting.Database.UseXXX to methods (#23354)

Replace #23350.

Refactor `setting.Database.UseMySQL` to
`setting.Database.Type.IsMySQL()`.

To avoid mismatching between `Type` and `UseXXX`.

This refactor can fix the bug mentioned in #23350, so it should be
backported.

models/unittest/testdb.go

@@ -76,7 +76,7 @@ func MainTest(m *testing.M, testOpts *TestOptions) {
 	setting.SSH.BuiltinServerUser = "builtinuser"
 	setting.SSH.Port = 3000
 	setting.SSH.Domain = "try.gitea.io"
-	setting.Database.UseSQLite3 = true
+	setting.Database.Type = "sqlite3"
 	setting.Repository.DefaultBranch = "master" // many test code still assume that default branch is called "master"
 	repoRootPath, err := os.MkdirTemp(os.TempDir(), "repos")
 	if err != nil {