Remove U2F support (#20141)

- Completely remove U2F support from 1.18.0, 1.17.0 will be the last
release that U2F is somewhat supported. Users who used U2F would already
be warned about using U2F for a while now and should hopefully already
be migrated. But starting 1.18 definitely remove it.

docs/content/doc/features/comparison.zh-tw.md

@@ -50,7 +50,7 @@ menu:
 | 有寫入權限的儲存庫 Token | ✓                                                  | ✘    | ✓         | ✓         | ✓         | ✘         | ✓            |
 | 內建 Container Registry  | [✘](https://github.com/go-gitea/gitea/issues/2316) | ✘    | ✘         | ✓         | ✓         | ✘         | ✘            |
 | 對外部 Git 鏡像          | ✓                                                  | ✓    | ✘         | ✘         | ✓         | ✓         | ✓            |
-| FIDO U2F (2FA)           | ✓                                                  | ✘    | ✓         | ✓         | ✓         | ✓         | ✘            |
+| FIDO (2FA)               | ✓                                                  | ✘    | ✓         | ✓         | ✓         | ✓         | ✘            |
 | 內建 CI/CD               | ✘                                                  | ✘    | ✓         | ✓         | ✓         | ✘         | ✘            |
 | 子群組: 群組中的群組     | ✘                                                  | ✘    | ✘         | ✓         | ✓         | ✘         | ✓            |