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>

tests/integration/api_packages_rpm_test.go

@@ -20,7 +20,6 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	rpm_module "code.gitea.io/gitea/modules/packages/rpm"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/ProtonMail/go-crypto/openpgp"
@@ -99,7 +98,7 @@ gpgcheck=1
 gpgkey=%sapi/packages/%s/rpm/repository.key`,
 					strings.Join(append([]string{user.LowerName}, groupParts...), "-"),
 					strings.Join(append([]string{user.Name, setting.AppName}, groupParts...), " - "),
-					util.URLJoin(setting.AppURL, groupURL),
+					strings.TrimSuffix(setting.AppURL, "/")+groupURL,
 					setting.AppURL,
 					user.Name,
 				)

tests/integration/api_repo_git_tags_test.go

@@ -14,7 +14,6 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/gitrepo"
 	api "code.gitea.io/gitea/modules/structs"
-	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/tests"
 
 	"github.com/stretchr/testify/assert"
@@ -58,7 +57,7 @@ func TestAPIGitTags(t *testing.T) {
 	assert.Equal(t, aTagMessage+"\n", tag.Message)
 	assert.Equal(t, user.Name, tag.Tagger.Name)
 	assert.Equal(t, user.Email, tag.Tagger.Email)
-	assert.Equal(t, util.URLJoin(repo.APIURL(), "git/tags", aTag.ID.String()), tag.URL)
+	assert.Equal(t, repo.APIURL()+"/git/tags/"+aTag.ID.String(), tag.URL)
 
 	// Should NOT work for lightweight tags
 	badReq := NewRequestf(t, "GET", "/api/v1/repos/%s/%s/git/tags/%s", user.Name, repo.Name, commit.ID.String()).