Update bazel to 6.3.2

Demote the errorprone pattern PreferredInterfaceType aka
MutableMethodReturnType to ensure the build still succeeds.

Change-Id: I6a6e44362e6ff943a26553890a2c84dbfe65128b

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index 0062ac9..91e4a9f 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-5.0.0
+6.3.2

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 93b779e..8bb4108 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -69,6 +69,7 @@
         "-Xep:MissingFail:ERROR",
         "-Xep:MissingOverride:ERROR",
         "-Xep:MutableConstantField:ERROR",
+        "-Xep:MutableMethodReturnType:WARN",
         "-Xep:NarrowingCompoundAssignment:ERROR",
         "-Xep:NonAtomicVolatileUpdate:ERROR",
         "-Xep:NonOverridingEquals:ERROR",