ocaml: update to ubunut:24.04 to fix build error

Fixes build error that happens with older versions of ocaml:
    ocamlopt -a types.ml reader.ml printer.ml env.ml core.ml -o mal_lib.cmxa
    File "env.ml", line 15, characters 8-21:
    Error: Unbound value Data.find_opt

impls/ocaml/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:xenial
+FROM ubuntu:24.04
 MAINTAINER Joel Martin <github@martintribe.org>
 
 ##########################################################
@@ -9,7 +9,8 @@ MAINTAINER Joel Martin <github@martintribe.org>
 RUN apt-get -y update
 
 # Required for running tests
-RUN apt-get -y install make python
+RUN apt-get -y install make python3
+RUN ln -fs /usr/bin/python3 /usr/local/bin/python
 
 # Some typical implementation and test requirements
 RUN apt-get -y install curl libreadline-dev libedit-dev