Conditionally set billing subheader for annual and biennial plans #6276515

Changes proposed in this Pull Request

The billing subheader was written just for annual plans and is not accurate on biennial plans.

Before, biennial plan showed ‘billed annually’:

After, biennial plan correctly shows ‘billed every two years’:

Testing instructions

Test the bug:

• Add plan to checkout
• Change to two year plan
• Proceed through checkout
• Go to https://wordpress.com/plans/[site]
• See billing subheader is stating as ‘billed annual’

Test the fix:

• Do the above steps with this PR
• Verify that two year plan shows ‘billed every two years’ in billing subheader

Fixes #54355