Add BTree::append() change to 1.96.0 relnotes

This change affected someone's code, reported in #157436.

I copied the text from #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

Author: theemathas

RELEASES.md

@@ -77,6 +77,7 @@ Compatibility Notes
 - [For `export_name`, `link_name`, and `link_section` attributes, if multiple of the same attribute is present, the first one now takes precedence.](https://github.com/rust-lang/rust/pull/153041)
 - [Update the minimum external LLVM to 21](https://github.com/rust-lang/rust/pull/153684)
 - On `avr` targets, C's `double` type is 32-bit by default, so [change `c_double` to `f32` on `avr` targets to match](https://github.com/rust-lang/rust/pull/154647). This is a breaking change, but necessary to make `c_double` match C's double.
+- [`BTreeMap::append()` was optimized, which may now cause panics for types with incorrect `Ord` impls](https://github.com/rust-lang/rust/pull/153107)
 
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