Move back to continuously updating Bazel. (#302)

We switched to the released version to minimize rebuildds of the
toolchain, but with downloading the toolchain this isn't a significant
issue any more. And without this, Bazel doesn't run on ARM macOS.

This may hit issues with broken Bazel builds and have to be reverted.
Added a TODO to reverse this as soon as we can anyways.

Also cleans up the old `.bazelversion` in favor of just using
`.bazeliskrc`. They both work, but the RC file comes first in the
sequence so happy to prefer it here.

.bazeliskrc

@@ -2,4 +2,5 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-USE_BAZEL_VERSION=latest
+# TODO: Return to a released version as soon as M1 macOS is supported.
+USE_BAZEL_VERSION=last_downstream_green

.bazelversion

@@ -1 +0,0 @@
-latest