Update the Mautic Partners policy to reflect governance model changes

[RCheesley]

With the introduction of the new governance model in December 2023, we neglected to update the Partner Policy to take into account Corporate membership being the primary financial requirement.

This PR:

• Removes references to sponsoring and using the big mac index to identify minimum cost for companies - instead we state they must be a corporate member of Mautic
• Removes references to GitHub Sponsors as we want to eventually move away from that due to the excess fees they charge
• Adds note about terms for grandfathered companies who were already sponsoring monthly at the equivalent rate, and the 12 month period they have to transition to being a corporate member

[escopecz]

Reads good to me 👍

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