Remove hardcoded runtimes

[svidhani]

As new runtimes get released by AWS (and other cloud platforms) we cannot have hardcoded values in our codebase. We need to come up with a better way(config for eg.) to support the runtimes.

[bleggett]

@svidhani and I were talking as a consequence of #366 , and we both think that we should simply stop maintaining a list of what core services target which runtime (see for instance line 28-30 of modifyCodebase.sh and should simply force all core services to use one runtime, defined in one variable, preferably the latest LTS.

I cannot see a good reason to avoid doing this that does not boil down to "well but then we would have to keep our core functions up to date"