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Rollup merge of #157556 - theemathas:btree-append-1.96.0-relnotes, r=Mark-Simulacrum
Add `BTree::append()` change to 1.96.0 relnotes
This change affected someone's code, reported in https://github.com/rust-lang/rust/issues/157436.
I copied the text from https://github.com/rust-lang/rust/issues/157496, but I figured that it belonged to the "Compatibility Notes" section, not "Internal Changes".
See discussion at https://rust-lang.zulipchat.com/#narrow/channel/241545-t-release/topic/Procedure.20for.20modifying.20past.20release.20notes/with/600817427
