[Feature] Update action to use node 20

[danielmarcano]

Related to Issue #9895

The following changes have been made because GitHub Actions will no longer support Node.js v12, as mentioned by @williamjallen in the linked issue. You can find the relevant information in the blog post referenced in that issue.

1. Updated action.yml so that it uses Node.js version 'node20'
2. Updated README.md to reflect this change
3. Updated test.yml matrix node-version array to [20.x]
4. Updated version to 2.0.0 in package.json to reflect this change, as this is a breaking change, according to the rules of semantic versioning
5. Upgraded @types/node version to "^20.8.7" to match Node.js v20 version

I have conducted a search for other repositories using Node.js v12 and identified the following three repositories, so I have opened other PRs updating Node.js to v20 in them:

• pdf-annotate
• peter-evans-create-pull-request
• peter-evans-repository-dispatch

Checks:

[x] Have run npm run lint and npm run test as specified in the README.md

P.S. If this PR is accepted, could you please add the "hacktoberfest-accepted" label to it, so that it counts towards my Hacktoberfest contributions?

[MasterOdin]

There is also the @types/node devDependency that should be upgraded to the same major version so that developers can use whatever new syntax features are allowed in that version.

[danielmarcano]

Hi, @MasterOdin! I have just updated the PR, including all the changes you have mentioned.

If you consider that it's good to go, could you please add the "hacktoberfest-accepted" label to it?

Thanks for everything ✨