Add .bazelversion, set to 5.1.1

Change-Id: I3d08ae5b2259799210ad8048220ff032c35a1741
diff --git a/.bazelversion b/.bazelversion
new file mode 100644
index 0000000..ac14c3d
--- /dev/null
+++ b/.bazelversion
@@ -0,0 +1 @@
+5.1.1