[skip ci] Mention AVX(2) detection fix for MSVC in UPGRADING

This is likely more important than the possibility to build for AVX-512
since our official binaries have build time support for SSE2 only.  So
proper detection of AVX may make a huge performance difference for
`base64_*()` and some MBString functionality.

Closes GH-16248.

Author: cmb69

UPGRADING

@@ -1171,6 +1171,8 @@ PHP 8.4 UPGRADE NOTES
 * Building with Visual Studio now requires at least Visual Studio 2019 (Visual
   Studio 2022 is recommended, though).
 
+* AVX(2) CPU support is now properly detected for MSVC builds.
+
 * Native AVX-512 builds are now supported (--enable-native-intrinsics=avx512).
 
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