Plans grid: copy changes after Monthly plans

[ciampo]

What

While working on #48963, we had to re-use some existing text strings because the ones requested in the designs were not already translated

Why

We should open a new PR with the exact copy requested in the design, and merge it once the translations are available

A/C

• PR should have the String freeze label, and be merged only when all translations are available
• Toggle component: From "Pay monthly" / "Pay annually" to "Monthly" / "Annually"
• Plan item / card (accordion variant)
  • From "billed annually" to "billed as %s annually" (since the price already has "/month")
  • From "per month, billed monthly" to "billed monthly" (since the price already has "/month")
• Plan item / card (table variant): From "billed annually" to "per month, billed as %s annually"
• Final checkout step of Step by Step flow: From "billed annually" to "per month, billed as %s annually"