[BPF] Add default cpu change in ReleaseNotes (llvm#131691)

The pull request [1] changed bpf default cpu from -mcpu=v1 to
-mcpu=v3 in clang20. Recently in [1], Yuval Deutscher suggested
to add an entry to clang20 ReleaseNotes so users can easily find
the change from documentation.

  [1] llvm#107008

Co-authored-by: Yonghong Song <[email protected]>

Author: yonghong-song

clang/docs/ReleaseNotes.rst

@@ -1300,6 +1300,11 @@ AVR Support
 
 - Reject C/C++ compilation for avr1 devices which have no SRAM.
 
+BPF Support
+^^^^^^^^^^^
+
+- Make ``-mcpu=v3`` as the default.
+
 DWARF Support in Clang
 ----------------------