Config: Revert map[string]string change for server settings

[roosterfish]

This reverts the change of the server config from map[string]any to map[string]string performed in #13373 as it breaks the lxc client when talking to older servers that still return a map[string]any on the clients GetServer() call.

See the reproducer steps here #13373 (comment).

It still keeps the bits from #13373 that removed the hidden configuration option as those changes are not connected with each other.

An option would have been to adapt the client to be able to talk with both the newer and older server but users of the pure REST API are also affected by this as config settings that were originally an int or bool are now returned as a string.
Contrary when setting the value they always have to be provided as a string in the JSON payload.

To not break the API we have to keep map[string]any.

[github-actions]

Heads up @ru-fu - the "Documentation" label was applied to this issue.

[roosterfish]

@MusicDin using your reproducer I can confirm this unblocks the failing client when using a 5.0.3 server with the core.trust_password setting:

require github.com/canonical/lxd v0.0.0-20240523230039-b975d5778417

replace github.com/canonical/lxd => /home/julian/dev/lxd // changes from this PR

julian@thinkpad:~/dev/test/lxdtest$ go run main.go
Server version: 5.0.3
Instance name: v1
Instance arch: x86_64
Instance type: virtual-machine
Instance stat: Running

[MusicDin]

Yep. Thank you very much 🚀