docs: use x402 v2 header names (PAYMENT-REQUIRED, PAYMENT-SIGNATURE, PAYMENT-RESPONSE)

[notorious-d-e-v]

The MPP vs x402 comparison page (added in #624) and the x402 upgrade guide reference x402 v1 legacy header names. x402 v2 dropped the X- prefix and renamed the credential header to PAYMENT-SIGNATURE. This updates both pages to the current spec.

v1 (legacy)	v2 (current)
X-PAYMENT-REQUIRED	PAYMENT-REQUIRED
X-PAYMENT	PAYMENT-SIGNATURE
X-PAYMENT-RESPONSE	PAYMENT-RESPONSE

[vercel]

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[chopmob-cloud]

+1 on adopting the x402 v2 header names. AlgoVoi's gateway has been shipping these in production since our canonical refactor last week: PAYMENT-REQUIRED, PAYMENT-SIGNATURE, and PAYMENT-RESPONSE (no X- prefix) on /mpp/probe and /r/{tenant}/{resource} across all 8 supported chains (Algorand, VOI, Hedera, Stellar, Base, Solana, Tempo, ARC testnet), content-negotiated against the canonical x402 v2 envelope.

Worth noting for anyone landing on the comparison page later: the v2 headers are byte-stable against the canonical envelope shape from the working-group convergence on x402-foundation/x402#2322 and #2334, so updating MPP's guidance here keeps the cross-protocol surface aligned with where upstream is settling.

[brendanjryan]

Thank you! This LGTM

[SkinGem]

Thank you