fix(gatsby-plugin-mdx): Use latest for acorn's ecmaVersion

[LekoArts]

Description

#38104 showed that you can't use some modern JS syntax in MDX layout components. This is most probably due to us using 2020 in the acorn parser.

The docs at https://github.com/acornjs/acorn/tree/master/acorn/ say:

ecmaVersion: Indicates the ECMAScript version to parse. Must be either 3, 5, 6 (or 2015), 7 (2016), 8 (2017), 9 (2018), 10 (2019), 11 (2020), 12 (2021), 13 (2022), 14 (2023), or "latest" (the latest the library supports). This influences support for strict mode, the set of reserved words, and support for new syntax features.

I think we should be fine moving it to latest as we use https://babeljs.io/docs/babel-preset-env which also always defaults to "latest".

Documentation

#38104 is handling docs

Tests

Existing tests should pass

Related Issues

N/A