docs: add S2 age filter changelog for PR 1528

changelog.d/1528-s2-ghostmaps-age-filter.md

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+- Add an adjustable event-age filter across S2 GhostMaps products, defaulting
+  to seven days with 1-, 3-, 14- and 30-day choices and an All markers option.
+  Preserve undated features, save the selected window and expire old events
+  while the map stays open. Thanks @dylanmaniatakes (#1528).