Update compiler target version in gulp and extension tsconfig

[norascheuch]

Update compiler target version in gulp and extension tsconfig to adjust it to our node version.

Tested locally (a couple of normal Query runs with Run Query and Run Query on Multiple Databases)

All in all we have 7 relevant tsconfig files, 2 are adjusted here, 3 others already use es6, 2 extend a config that uses es6.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[elenatanasoiu]

Just mentioning that we checked that VSCode uses es2021 as well: https://github.com/microsoft/vscode/blob/main/src/tsconfig.json#L8

[alexet]

The vscode version we claim we support doesn't target es2021. So I am not sure this is correct at least not with a minimum version bump.