Fix APT keyring location in Linux installation instructions

[borfast]

The current installation instructions mention /usr/share/keyrings as the location to copy the apt key files to, but according to the sources.list and apt-key manpages, the correct location for manually installed keys would be /etc/apt/keyrings.

This PR changes the instructions to point to that location instead.

From the sources.list manpage:

The recommended locations for keyrings are /usr/share/keyrings for keyrings managed by packages, and /etc/apt/keyrings for keyrings managed by the system operator.

From the apt-key manpage:

FILES
/etc/apt/trusted.gpg
Keyring of local trusted keys, new keys will be added here. Configuration Item: Dir::Etc::Trusted.

/etc/apt/trusted.gpg.d/
File fragments for the trusted keys, additional keyrings can be stored here (by other packages or the administrator). Configuration Item Dir::Etc::TrustedParts.

/etc/apt/keyrings/
Place to store additional keyrings to be used with Signed-By.

[cliAutomation]

Hi! Thanks for the pull request. Please ensure that this change is linked to an issue by mentioning an issue number in the description of the pull request. If this pull request would close the issue, please put the word 'Fixes' before the issue number somewhere in the pull request body. If this is a tiny change like fixing a typo, feel free to ignore this message.

[williammartin]

@borfast do you know @hongquan from #8693 or did these docs just change recently? Pretty coincidental that these were opened within a day of each other!

[borfast]

@williammartin, no, I don't know them, it was just a coincidence 😅
The standard for the keyrings location has changed quite some time ago. For example, I opened a similar PR for another OSS project almost a year ago, and at that time it was already "old news".

Also, apologies for the duplicate PR. I did search to see if there was something already but I didn't see #8693. Feel free to close this one, if you prefer to do it all in one go with the other one.

[williammartin]

That's so funny. Alright I think I'll close this one then, thank you for basically doing a review of this part of the other PR on my behalf 😅

[borfast]

Glad I could help 😂