Update MPP overview page for v0.7 push mode change

[oceans404]

Page to update: https://developers.stellar.org/docs/build/agentic-payments/mpp
SDK PR for context: stellar/stellar-mpp-sdk#47

What to change

Under "Supported MPP Intents > Charge", the push mode bullet currently reads:

Push — The client broadcasts the transaction itself and sends the transaction hash for server verification.

This is stale as of v0.7. Update it to reflect the new credential format:

Push — The client broadcasts the transaction itself and sends a signedHash credential for server verification: the transaction hash plus a signature proving the client controls the paying address. rejectUnsignedPush defaults to true as of v0.7; servers that need to temporarily accept the legacy unsigned hash must explicitly set rejectUnsignedPush: false.

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