doc: Changelog entry

Author: effigies

changelog.d/20250828_161028_markiewicz_deprecate_89plus.md

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+
+### Added
+
+- Issue deprecation warning for `"89+"` in `age` columns, per
+  [bids-standard/bids-specification#2162][].
+
+[bids-standard/bids-specification#2162]: https://github.com/bids-standard/bids-specification/pull/2162
+
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+### Changed
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