Replace .bazelversion files with symlinks

Replaces all nested `.bazelversion` files with symbolic links to the
top-level `.bazelversion` file. Removes `scripts/sync-bazelversion.sh`.

Also removes `dt_patches/compiler_sources/.bazel{rc,version}` since
they're not required by any tests.

---

I missed updating the `.bazelversion` files in bazel-contrib#1778 because I forgot to
run `scripts/sync-bazelversion.sh`. I'd added this script previously to
avoid creating symlinks that might break Windows users that don't have
symlinks enabled. At the same time, `.bazelversion` doesn't have an
`import ../.bazelversion` directive like `.bazelrc` does.

However, it seems Windows has supported symlinks for a while, and users
shouldn't have any trouble enabling them on their workstations. We also
already use them for `protobuf.patch` files throughout the nested repos.
So it's time to use symlinks for `.bazelversion` files as well, to
ensure these files don't fall out of sync again.

Author: mbland

dt_patches/compiler_sources/.bazelrc

@@ -0,0 +1 @@
+../../.bazelversion

dt_patches/compiler_sources/.bazelversion

@@ -0,0 +1 @@
+../../.bazelversion

dt_patches/test_dt_patches/.bazelversion

@@ -0,0 +1 @@
+../../.bazelversion

dt_patches/test_dt_patches/.bazelversion

@@ -0,0 +1 @@
+../../.bazelversion