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>

go.sum

@@ -951,8 +951,6 @@ mvdan.cc/xurls/v2 v2.6.0 h1:3NTZpeTxYVWNSokW3MKeyVkz/j7uYXYiMtXRUfmjbgI=
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-strk.kbt.io/projects/go/libravatar v0.0.0-20260301104140-add494e31dab h1:3IZDVyI8uBmZko6pwm39f7mE0aTY7LViNdQHMeH7U60=
-strk.kbt.io/projects/go/libravatar v0.0.0-20260301104140-add494e31dab/go.mod h1:FJGmPh3vz9jSos1L/F91iAgnC/aejc0wIIrF2ZwJxdY=
 xorm.io/builder v0.3.13 h1:a3jmiVVL19psGeXx8GIurTp7p0IIgqeDmwhcR6BAOAo=
 xorm.io/builder v0.3.13/go.mod h1:aUW0S9eb9VCaPohFCH3j7czOx1PMW3i1HrSzbLYGBSE=
 xorm.io/xorm v1.4.1 h1:m7QlNd0eBGb31IV4Q/ow0Du83rtdC1CiwlvJZGvYde8=