Update Kotlin to 2.1.20

[lppedd]

2.1.20 will solve a bunch of blockers spawned from the K1 > K2 transition, and I feel like it's the right compromise to update to.
Anything prior to 2.1.20 is half broken or unreliable for K/JS.

@petertrr I'll open a PR once it's out and I've managed to upgrade other libs without issues.

[morisil]

@lppedd see #131

[petertrr]

Solved by #120, let me know (or create follow-up PRs) if I forgot anything

[lppedd]

@petertrr seems ok, thanks! I'll check better once I get some free time.