Add changelogs

Author: amesgen

ouroboros-consensus/changelog.d/20250626_193647_alexander.esgen_ledgerdb_garbage_collect_states.md

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+### Breaking
+
+- Changed pruning of immutable ledger states to happen on LedgerDB garbage
+  collection instead of directly on every block adoption. This is purely an
+  internal refactoring (with breaking API changes) supporting predictable ledger
+  snapshotting.
+
+- Avoid maintaining volatile ledger states during ledger replay, making it
+  slightly more efficient.