Bump minimal node version from 6 to 8.10.0

What does this MR do?

• Enforces the node version via the engines flag in package.json
• Updating developer/install documentation
• Remove QA job for node@6

I have chosen a minimum of 8.10.0, as it is the minimum that eslint@5.9.0 has specified.

What are the relevant issue numbers?

Closes #54065 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer