fix(avatar): use sha256 and inline the federated avatar lookup (#38843)

- Hash emails with sha256. Gravatar moved to sha256, and both it and
libravatar.org serve the same image for either hash.
- Drop `strk.kbt.io/projects/go/libravatar` for a 46 line inline SRV
lookup. It could not bound or cancel its DNS query and panicked on an
unexpected resolver error. The replacement carries the request context
and a 3s timeout.
- Fix federated avatars querying DNS for every avatar on every render.
`loadAvatarSetting` compared a cache field that was never assigned, so
each call rebuilt the resolver and dropped its cache. That cache is
gone, both settings are read where they are used.
- Migration 348 recreates `email_hash` with a 64 char hash column and a
`hash_type` column, so a later algorithm change can tell old rows apart.
The MD5 rows are unreachable and their `UNIQUE` email index would reject
the SHA256 replacements.
- Fix a re-saved avatar form replacing an uploaded avatar with a random
one.
- Remove the `duoshuo` `GRAVATAR_SOURCE` alias, that service shut down
in 2017.
- Remove dead i18n key.

Fixes: https://github.com/go-gitea/gitea/issues/34284
Fixes: https://github.com/go-gitea/gitea/issues/28110
Docs: https://gitea.com/gitea/docs/pulls/499
Signed-off-by: silverwind <me@silverwind.io>

custom/conf/app.example.ini

@@ -2038,8 +2038,7 @@ LEVEL = Info
 ;; If the uploaded file is not larger than this byte size, the image will be used as is, without resizing/converting.
 ;AVATAR_MAX_ORIGIN_SIZE = 262144
 ;;
-;; Chinese users can choose "duoshuo"
-;; or a custom avatar source, like: http://cn.gravatar.com/avatar/
+;; "gravatar", "libravatar", or any other Gravatar-compatible source, like: https://cn.gravatar.com/avatar/
 ;GRAVATAR_SOURCE = gravatar
 ;;
 ;; Deprecated, see Web UI Admin Panel -> Config -> Settings