settings: file: change FS (or file system) wording to File

Currently there is inconsistency in repository file names, APIs, Kconfig
options and documentation around file / file-system backend for settings
storage, as both "file" and "FS (file system)" are used. As an example,
there is `CONFIG_SETTINGS_FS` Kconfig option, but the file that implements
this settings backend is called `settings_file.c`. Another example are
names of static functions that implement settings storage API:
`settings_file_load()`, `settings_file_save()` and
`settings_fs_storage_get()`.

This backend is actually storing all settings in a single file, so it makes
sense to use "File" as the name of backend, instead of a more general
"FS" (which would make sense if several files would be used to store
settings).

Fix inconsistency in used wording in the tree and unify it to "settings
file backend". This naming is more precise to how the implementation looks.
It will also make it easier to grep through the codebase and analyze
existing code.

Deprecate settings_mount_fs_backend() function and all Kconfig options
starting with `CONFIG_SETTINGS_FS`.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>