Split buttons and update copy on Logged out navs

What does this MR do and why?

We need to fix discrepancies in our Logged out navigation bar:

• Pattern: Use 2 different buttons for Sign in and Register instead of a combined Sign in / Register button
• Copy: For SaaS we should refer use Register and Sign in instead of Sign up now and Log in Signing see https://docs.gitlab.com/ee/development/documentation/styleguide/word_list.html#sign-in and #352143 (closed) for reference.

Screenshots or screen recordings

Split buttons for S-M

Before	After (Sign up enabled)	After (Sign up disabled)
Sign in / Register	Register, Sign in	Sign in

SaaS copy change

Before	After
Sign up now, Log in	Register, Sign in

How to set up and validate locally

Button split

1. Sign out of your gdk
2. Navigate to a public project make sure you see both Register and Sign in
3. Sign in to your instance
4. Go to Admin > Settings > Sign up and disable sign up
5. Repeat step 1 and 2, make sure you only see Sign in this time

Copy changes

1. Simulate a SaaS instance
2. Sign out of your gdk
3. Navigate to a public project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #379750 (closed)