Deprecate legacy installer (#1014)

<img width="799" alt="image"
src="https://github.com/databrickslabs/ucx/assets/259697/2aa5fed6-5734-44c2-87bc-39fbc214d5fa">

install.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-# This script will eventually be replaced with `databricks labs install ucx` command.
+echo -e "\033[0;31m\033[1m!!!!!!!!!!!!!!!\n!!! DEPRECATED: This script is going to be removed soon. See https://github.com/databrickslabs/ucx#installation\n!!!!!!!!!!!!!!!\033[0m"
 
 # Initialize an empty array to store Python binary paths
 python_binaries=()
@@ -84,4 +84,4 @@ $py -m pip install --quiet -e .
 # without console_scripts entrypoint
 $py -m databricks.labs.ucx.install
 
-rm -r "$tmp_dir"
+rm -r "$tmp_dir"