AI Tiered Plans: Update upgrade button from Usage Panel to support tiered plans

[lhkowalski]

Similiar to #33361, but for the upgrade button present on the Usage Panel.

The current state of the button is:

But it needs to mention the next tier available for the site, based on the current one.

It also needs to handle properly the checkout URL for Simple sites.

[lhkowalski]

I checked the Figma files, and the expected labels are:

• when the current tier is free, "Upgrade"
• when the current tier is 100, "Get 200 requests"
• when the current tier is 200, "Get 500 requests"
• when the current tier is 500, "Get 750 requests" (this is not on the designs, but since we added a few new plans, for inference this is the expected label)
• when the current tier is 750, "Get 1000 requests" (same as above)
• when the current tier is 1000 (the last tier), "Get more requests" (1000 is now the last tier instead of 500, so the original expectation for the 500 plan is applied here)

[lhkowalski]

The upgrade button text is updated, and now we need to make it point to the right place when running on Simple sites. I am waiting on the approach from #33361 so we can use the same code.

[lhkowalski]

#34254 solved the checkout URL issue for Simple sites on the Usage Panel as well.

The last missing part is handling the "Contact Us" call-to-action when the user is on the last tier. This is being handled correctly on Jetpack sites, so we just need to add an extra exception to show the correct call to action on Simple sites as well.

I am working on it.