Update capitalization of First-party Mode

[benbowler]

Feature Description

We should keep the naming convention the same across the plugin, i.e., in some parts, we name it correctly as First-party mode, but in others, we have called it First-Party Mode, i.e., in the Site Health output. For Settings, we have it as a title case in First-party Mode.

It should be consistently in First-party Mode in the plugin.

@benbowler I just want to add in another situation whwere we need to change the title. From this Asana ticket.

The warning banner displayed when a user can't enable FPM (due to script access or health check) should say "First-party mode" instead of "first-party mode"

Note, the Figma designs have now be updated to represent this change.

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