Bump GraalVM Version to 20.3

[iamrecursion]

Summary

Oracle has just released the new version of GraalVM, and we need to update our project and build to use it.

Value

We won't get stuck on an older and unsupported version of GraalVM.

Specification

• Update the build to use 20.3.
• Fix all deprecations by updating the interpreter to use the replacements where necessary.
• Check if the MethodHandle support changes in the Native Image are enough to remove the substitutions we have.
  • Remove ReplacementStatics and ReplacementAkkaSpinWait and verify if the native images still build and pass the tests.

Acceptance Criteria & Test Cases

• The build uses the new version of Graal and there aren't any deprecation warnings.