Add changelog entry

Author: palas

cardano-testnet/changelog.d/20260226_005016_palas_default_to_update_timestamps.md

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+### Added
+
+- Added `--preserve-timestamps` flag. When set, genesis file timestamps are kept as-is.
+
+### Changed
+
+- Timestamps in genesis files are now updated to the current date by default.
+- The `--update-time` flag is now internal (hidden, kept for backward compatibility).
+