Update gitea-vet to check FSFE REUSE (#22004)

Related to:
- #21840
- https://gitea.com/gitea/gitea-vet/pulls/21

What it looks like when it's working:
https://drone.gitea.io/go-gitea/gitea/64040/1/5

All available SPDX license identifiers: [SPDX License
List](https://spdx.org/licenses/).

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/context/xsrf.go

@@ -13,6 +13,7 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+// SPDX-License-Identifier: Apache-2.0
 
 package context