fix(backend): Change isPublic to correctly recognize as:Public term

[hakatashi]

What

ActivityPub経由で投稿されたNoteがpublicであるかどうかを as#Public ではなく as:Public で識別するように修正

Why

ActivityPub経由で受信したNoteがpublicであるかの判定の1つに as#Public とありますが、これは as:Public の間違いであると思われます。

NOTE
Compacting an ActivityStreams object using the ActivityStreams JSON-LD context might result in https://www.w3.org/ns/activitystreams#Public being represented as simply Public or as:Public which are valid representations of the Public collection. Implementations which treat ActivityStreams objects as simply JSON rather than converting an incoming activity over to a local context using JSON-LD tooling should be aware of this and should be prepared to accept all three representations.
5.6 Public Addressing - ActivityPub

これに起因して、外部サーバーから as:Public で投稿されたパブリックな投稿がタイムラインに表示されない不具合が存在しますが、この変更によって修正されます。

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
  • No change to UI
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Merging #11347 (2444eeb) into develop (c1a19ff) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop   #11347   +/-   ##
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  Coverage    77.69%   77.69%           
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  Files          173      173           
  Lines        21599    21599           
  Branches       498      498           
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  Hits         16782    16782           
  Misses        4817     4817