Update .bazelversion from 7.6.0 to 8.6.0

Bazelisk walks up the directory tree to find .bazelversion, so this
file shadows tools/OpenROAD/.bazelversion (8.6.0) when working in
the OpenROAD submodule. This caused a cryptic module resolution error
instead of using the correct Bazel version.

Fixes: The-OpenROAD-Project/OpenROAD#9978

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

.bazelversion

@@ -1 +1 @@
-7.6.0
+8.6.0