docs: update supportPolicy table

[HonkingGoose]

Changes:

• Drop Node 10 from table as that is end of life
• Drop Node 15 from table as that is also end of life
• Add Node 16 to table (current release)
• Bump "current as of" to April

Context:

Closes #9551.

This PR should be merged on the 30th of April, as that's when the table matches the Node schedule.
Or you could merge it now, and live with the fact that it's a bit mismatched compared to the Node release schedule.

Source for schedule: https://nodejs.org/en/about/releases/

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added unit tests, or
• No new tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[viceice]

@HonkingGoose is data/node-js-schedule.json also uptodate?

[HonkingGoose]

@HonkingGoose is data/node-js-schedule.json also uptodate?

Yes, we're up to date with their upstream file. The upstream last changed their file on the 6th of November, and I made PR #8508 to get us up to date with that version.

EDIT: link to upstream file https://github.com/nodejs/Release/blob/main/schedule.json

So I don't think I need to do anything else right now.

[renovate-release]

🎉 This PR is included in version 25.1.2 🎉

The release is available on:

• GitHub release
• 25.1.2

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