Update ecmaVersion to 'latest' in esnext.js in eslint-plugin to ensure it's esnext.

[iamyoki]

Description

This PR updates a simple config in esnext.js in eslint-plugin, it prevents some potential problems.
It updates

  parserOptions: {
    ecmaVersion: '2018',
    sourceType: 'module',
  }

to

  parserOptions: {
    ecmaVersion: 'latest',
    sourceType: 'module',
  }

so to ensure it always be esnext latest version.

Here is the official docs mentioned: https://eslint.org/docs/user-guide/configuring/language-options#specifying-parser-options.

Also updates env es6 to es2021.

Type of change

• Patch: Bug (non-breaking change which fixes an issue)
• Minor: New feature (non-breaking change which adds functionality)
• Major: Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added a changelog entry, prefixed by the type of change noted above (Documentation fix and Test update does not need a changelog as we do not publish a new version for these changes)

[iamyoki]

CLA assigned. Please reruns..

[lucabezerra]

@iamyoki Thanks for the contribution and apologies for the delay. Do you mind adding an entry in the corresponding CHANGELOG.md, under the Unreleased > Changed section?

[iamyoki]

@lucabezerra No problem.

[lucabezerra]

@iamyoki Actually I meant under the Unreleased > Changed section. I think you might have an outdated version of the repo, but if you look at the current version of that file, the Unreleased section is uncommented and actually has an entry in it already :)

[iamyoki]

@lucabezerra Yeah, my mistake, fixed.

[lucabezerra]

@iamyoki I think you added it twice by mistake, heh :)

[iamyoki]

@lucabezerra Hah, finally.

[BPScott]

latest isn't supported in our the parsers versions that we currently require.

Let's have use ecmaVersion: 2021 to get us onto the latest version in this PR

Then if you're feeling a brave, open new PRs for replacing babel-eslint with @babel/eslint-parser, and updating @typescript-eslint/parser to the latest version in the v4 branch (we'll want to go to v5 at some point but perhaps we should do that as part of the plugin updates required to support eslint v8).