Draft: Update maintenance policy docs to reflect the extended maintenance policy
What does this MR do and why?
Addressing gitlab-com/gl-infra/delivery#21183
We're extending the maintenance policy for backporting bug fixes to be aligned with the procedure for backporting security fixes in gitlab-com/gl-infra&971 (closed). This means that all bug and security fixes will be backported to the maintained versions (current + 2 previous gitlab versions). This MR updates the maintenance policy documentation to reflect the extended maintenance policy.
Keep it in draft until the extension is rolled out.
References
gitlab-com/gl-infra/delivery#21183
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