chore: Update tsconfig target to ES2021

[TrevorBurnham]

This PR updates the build target from ES2015 to ES2021, the newest ECMAScript version supported by all target browsers. This reduces the bundle size by 3.7%, with the potential for further improvement if code is updated to take advantage of newly-supported language features, including:

• async/await
• Object.assign, Array.includes, String.startsWith/endsWith
• const/let block scoping
• arrow functions
• template literals
• Optional chaining (?.)
• Nullish coalescing (??)
• Logical assignment operators** (&&=, ||=, ??=)

Here's the corresponding PR to bump the ECMAScript target in the components repo: cloudscape-design/components#3801

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[just-boris]

Looks good overall, but it will require an internal sanity check as well.

Let's finish the resize observer story first and then get on this one

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (193e04b) to head (d4f69c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #163   +/-   ##
=======================================
  Coverage   98.78%   98.78%           
=======================================
  Files          36       36           
  Lines         904      904           
  Branches      243      255   +12     
=======================================
  Hits          893      893           
  Misses         11       11           

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[just-boris]

Time to come back to this PR.

Pushed it internally and did a dry-run: 7572079946

There are too many consumer packages unable to understand optional chaining. I opened a follow up CR to try raising at least up to 2019

#169

[TrevorBurnham]

There are too many consumer packages unable to understand optional chaining.

Oof. Webpack 4 just won't die. 😩