switch destination directory for apt signing keys (#28639) (#28642)
Backport #28639 by @denyskon
According to [Debian
docs](https://wiki.debian.org/DebianRepository/UseThirdParty):
> The certificate MUST NOT be placed in /etc/apt/trusted.gpg.d or loaded
by apt-key add.
> ...
> If future updates to the certificate will be managed by an apt/dpkg
package as recommended below, then it SHOULD be downloaded into
/usr/share/keyrings using the same filename that will be provided by the
package. If it will be managed locally , it SHOULD be downloaded into
/etc/apt/keyrings instead.
> ...
> A sources.list entry SHOULD have the signed-by option set.
Co-authored-by: Denys Konovalov <kontakt@denyskon.de>
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