feat: Upgrade min target to ES2020

[benjackwhite]

Problem

We're making a major change so we considered other things we might want to do and this is one of them. Removes support for IE11 - if you need it, that's what posthog-js is for 😅

Changes

Fixes https://github.com/PostHog/posthog-js-lite/issues/163

• Upgrade target to ES2020

Release info Sub-libraries affected

Bump level

• [x] Major
• [ ] Minor
• [ ] Patch

Libraries affected

• [x] All of them
• [ ] posthog-web
• [ ] posthog-node
• [ ] posthog-react-native

Changelog notes

• Updates the minimum build target to ES2020 (essentially removing support for IE11)

[github-actions[bot]]

Size Change: 0 B 🆕

Total Size: 0 B

_{compressed-size-action}

[marandaneto]

Can we remove htis hack maybe? https://github.com/PostHog/posthog-js-lite/blob/a1d9091396682de005f448d11531c0425e0b18d5/posthog-node/src/feature-flags.ts#L26-L28