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>

services/convert/git_commit_test.go

@@ -11,7 +11,6 @@ import (
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/git"
 	api "code.gitea.io/gitea/modules/structs"
-	"code.gitea.io/gitea/modules/util"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -35,7 +34,7 @@ func TestToCommitMeta(t *testing.T) {
 	assert.NotNil(t, commitMeta)
 	assert.Equal(t, &api.CommitMeta{
 		SHA:     sha1.EmptyObjectID().String(),
-		URL:     util.URLJoin(headRepo.APIURL(), "git/commits", sha1.EmptyObjectID().String()),
+		URL:     headRepo.APIURL() + "/git/commits/" + sha1.EmptyObjectID().String(),
 		Created: time.Unix(0, 0),
 	}, commitMeta)
 }