That’s weird if you did not have this error before.
Can you confirm /etc/apt/trusted.gpg.d/yunohost-trixie.gpg does not exist ?
Disclaimer : I did the following steps on my server after a messy upgrade to Yunohost 13, and it worked. But I’m no expert (read : I have no real idea what I’m doing). Tip : backup any file before modifying them (cp file_i_m_gonna_break file_i_m_gonna_break.bak)
Try, from the server, downloading yunohost_trixie.asc from YunoHost Repository (for example with wget https://repo.yunohost.org/keys/yunohost_trixie.asc).
Put it in /etc/apt/trusted.gpg.d/.
Rename it to yunohost-trixie.asc (replace the underscore with a dash).
Then dearmor it with gpg --dearmor yunohost-trixie.asc.
The output file might be named yunohost-trixie.asc.gpg, in that case rename it to yunohost-trixie.gpg (to match the apt source file).
And try updating.