Remove transpiling, target es11/es2020

We currently try to transpile, add polyfills, and target older browsers. This is something we want to change, and just focus on targeting es2020 ([ref this comment](https://digdir.slack.com/archives/C07A822E9PT/p1749541859505669?thread_ts=1747917879.904519&cid=C07A822E9PT)). Targetting `es2020` happens already in #3231, but that might leave CoreJS and babel transpiling unused. We should remove it if we don't use it.

Start by reviewing the transpilation and polyfill configuration, including the CoreJS and Babel references mentioned in the issue, and compare it with the ES2020 targeting work in #3231. Done means unused transpiling and polyfill setup has been removed while the project continues targeting ES2020.

Written by the indexing model from the issue text.