Package supports Node >= 12 but CI only tests Node 20 and 26

As title states, engines.node in package.json is ">=12.0.0" but the CI only tests Node 20 and 26.

Bug fixes and new features could accidentally break functionality when using an older Node version, with no CI feedback. Either the test matrix in the CI workflow needs updating with old LTS versions (12, 14, 16, 18), or the engine version needs bumping (which would be a breaking change).