[CHORE] Say in the changelog that the unsynchronized callback marker is gone

AsyncData::callback_thread is on main today, read and written from two threads with
nothing ordering them and with both accessors marked no-thread-sanitizer, so removing it
is a fix a user of the released client gets rather than something internal to this
branch. The use after free this branch also fixes is not listed, because the code it
lives in does not exist outside this branch and a changelog entry would claim a repair to
something that never shipped.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

CHANGELOG.md

@@ -123,6 +123,10 @@ Increment the:
   including the ones the IO thread delivers, instead of waiting on a completion
   only that thread goes on to publish
   ([#4395](https://github.com/open-telemetry/opentelemetry-cpp/pull/4395))
+* [BUG] Decide whether a curl thread is inside a callback without an
+  unsynchronized member, which was read and written from two threads behind a
+  sanitizer suppression
+  ([#4395](https://github.com/open-telemetry/opentelemetry-cpp/pull/4395))
 * [CODE HEALTH] Enable clang-tidy `modernize-deprecated-headers` and replace
   deprecated C headers (`stdint.h`, `stddef.h`, `stdlib.h`, `string.h`,
   `stdio.h`, `ctype.h`, `limits.h`, `assert.h`) with their C++ equivalents