Lock down Bazel dependencies (#23)

Ran `bazel mod deps --lockfile_mode=update` to force Bazel to resolve
all dependencies. Hopefully this should stop unexpected changes here.
My editor seems to want to update some entries for no apparent reason.

Add a .bazelversion file telling Bazelisk to use the most recent
stable version of Bazel.

Both recommendations come from
https://bazel.build/external/lockfile#best-practices.

.bazelversion

@@ -0,0 +1 @@
+7.3.0