Use POST route consistently for phone creation

[aduth]

🛠 Summary of changes

This pull request aims to address some incomplete refactoring started in #7826 and work toward reconciling (and hopefully consolidating) phone creation routes.

In #7826, the new spam protection screen reused POST routes for phone creation with the expectation that we would remove PATCH as a supported verb, reflected in routes.rb on main:

identity-idp/config/routes.rb

Lines 254 to 255 in 3a3b3a9

	patch '/phone_setup' => 'users/phone_setup#create' # TODO: Remove after next deploy
	post '/phone_setup' => 'users/phone_setup#create'

However, we never updated the existing reference to the PATCH route in app/views/users/phone_setup/index.html.erb, which is updated here.

This also removes a data- attribute which, as best I can tell, is not used. (It was added in #3253, though was never used from what I can see)

📜 Testing Plan

1. Navigate to http://localhost:3000
2. Create a new account
3. Select phone as MFA
4. Observe no issues in submitting phone number entry page

[aduth]

I should flag that this creates some inconsistency with other MFA setup paths which still use PATCH as a verb. I personally don't think PATCH is the best verb to be using for adding new MFA methods, though I'm open to reconsidering.