Remove Node 20 support

[jescalada]

Node 20 is about to reach end-of-life soon:

Furthermore, a few dependabot PRs require bumping Node to 22:

#1435
#1444

@coopernetes Is there any reason to keep supporting Node 20? Just wondering since you added Node 20 to the dependency matrix back in December #1315

Checklist

• Remove Node 20 from the CI test matrix.
• Bump Node version requirement in docs (if they exist)
• Update/remove Node 20 usages from other CI files
• Close/update dependabot PRs listed above

Additional context
Discussed in #1416

[coopernetes]

Hey @jescalada no issue deprecating Node 20. The rationale was it was that Node 20 was the version that we baselined (loosely, not formally) based on our required engines in package.json. It's still receiving maintenance (security) patches so it seemed pragmatic at the time.

We should put in our CONTRIBUTING docs our policy for Node versions. That policy should baseline on either N-1 (latest LTS plus last LTS version) or N-2 (if we believe organizations would appreciate an older baseline since internal policy may not keep up with Node versions aggressively). We could even default to just N version of LTS once we release 2.0. A good time to move the Node version to 26 may be for 2.1.

[jescalada]

@coopernetes I opted to drop only 20 and keep latest and N-1 as a middle point. We can definitely add support for 26 as soon as it comes out. #1462

[coopernetes]

@jescalada wonder if we can re-open this temporarily to address these PRs?

#1445
#1436

EDIT: Merged the above two PRs to complete this Node version uplift.