Loosen restrictions on ES5 syntax in node_modules

[wylieconlon]

Now that IE11 is no longer supported by Kibana for future versions, the strict checking of node_modules that was implemented in #59972 could be made less strict. It might even be possible to remove all checks that were implemented. For reference, there is a compatibility table here which could indicate features beyond ES2015 that are missing full support in some environments.

cc @spalger

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)