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 99.32%. Comparing base (86f1d80) to head (62c2f84).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #163   +/-   ##
=======================================
  Coverage   99.32%   99.32%           
=======================================
  Files          36       36           
  Lines         893      893           
  Branches      248      235   -13     
=======================================
  Hits          887      887           
  Misses          6        6           

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