Document API SDK and Plugin SDK feature branch management in RELEASING.md

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As part of releasing SPIRE, we tag the API SDK and Plugin SDK repositories so folks know what's compatible with the SPIRE release they're using. We typically tag main, which is publicly released features ... features still in development live in next branch, which SPIRE tracks.

When a SPIRE release occurs, API changes for new features to be released may still be residing in the next branch. As part of the release, we need to ensure that those changes (if present) are cherry-picked into their respective main branch prior to tagging. The SPIRE release documentation should capture this part of the procedure, as it is currently missing