[landing] change request access back to subscribe for updates

Summary:
resolves the following request access copy touchup:

https://linear.app/comm/issue/ENG-4620/request-access-for-our-beta-today-←-we-dont-have-any-mechanism-to

Depends on D8781

Test Plan:
Please see screenshots:

Default states:

Success state:

Error state:

Reviewers: atul, kamil, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8782