Add release notes for bouncycastle update

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Co-authored-by: thll <886541+thll@users.noreply.github.com>

Author: Copilot

changelog/unreleased/pr-25684.toml

@@ -0,0 +1,4 @@
+type = "c"
+message = "Updated Bouncy Castle dependencies to `1.84`."
+
+pulls = ["25684"]