Remove util.URLJoin and replace all callers with direct path concatenation (#36867)

`util.URLJoin` was deprecated with unclear semantics (path normalization
via `url.Parse`/`ResolveReference` that surprised callers). This removes
it entirely and replaces all usages with straightforward `"/"` string
concatenation.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: wxiaoguang <2114189+wxiaoguang@users.noreply.github.com>

modules/util/util_test.go

@@ -11,39 +11,6 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func TestURLJoin(t *testing.T) {
-	type test struct {
-		Expected string
-		Base     string
-		Elements []string
-	}
-	newTest := func(expected, base string, elements ...string) test {
-		return test{Expected: expected, Base: base, Elements: elements}
-	}
-	for _, test := range []test{
-		newTest("https://try.gitea.io/a/b/c",
-			"https://try.gitea.io", "a/b", "c"),
-		newTest("https://try.gitea.io/a/b/c",
-			"https://try.gitea.io/", "/a/b/", "/c/"),
-		newTest("https://try.gitea.io/a/c",
-			"https://try.gitea.io/", "/a/./b/", "../c/"),
-		newTest("a/b/c",
-			"a", "b/c/"),
-		newTest("a/b/d",
-			"a/", "b/c/", "/../d/"),
-		newTest("https://try.gitea.io/a/b/c#d",
-			"https://try.gitea.io", "a/b", "c#d"),
-		newTest("/a/b/d",
-			"/a/", "b/c/", "/../d/"),
-		newTest("/a/b/c",
-			"/a", "b/c/"),
-		newTest("/a/b/c#hash",
-			"/a", "b/c#hash"),
-	} {
-		assert.Equal(t, test.Expected, URLJoin(test.Base, test.Elements...))
-	}
-}
-
 func TestIsEmptyString(t *testing.T) {
 	cases := []struct {
 		s        string