doc: Support end dates not aligned with https://ubuntu.com/about/release-cycle

The overall release cycle web page correctly states that legacy support will be 15 years but this is not reflected in the Canonical OpenStack docs.

Overall release cycle page:

Canonical OpenStack docs:

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Document: reference/release-cycle-and-supported-versions.rst

Open reference/release-cycle-and-supported-versions.rst and compare its support end dates with the Ubuntu release-cycle page linked in the issue. Update the documented legacy-support period so it matches the stated 15-year cycle, then review the rendered document to confirm the dates are consistent.

Written by the indexing model from the issue text.