Plans: update "Limited time offer" to "Special offer"

[michaeldcain]

When we switched the local currency discounts to use introductory offers in #86434, we consolidated the logic for the offer pill to use the same "Limited time offer" text as the Woo Express plans that were already utilizing introductory offers. @niranjan-uma-shankar then pointed out that since these promotions are longer-running, we shouldn't use the "limited time" wording (p58i-fkS-p2#comment-64883).

This PR update the text to use the wording "Special offer", which falls within the constraint of "unique to our normal business offerings". I understand that the original wording was "One time discount", but since this is the fallback wording for all introductory offers, "Special offer" felt more applicable. We can add conditions to use "One time discount" for these specific offers in a follow-up if it's needed.

To test:

• For the INR, PHP, or MXN currencies, on a site that hasn't previously had a plan purchase:
• Visit Upgrades > Plans
• Verify that the introductory offer pill says "Special offer"