bump tests/checks from 14,16 to 16,18

[purplecabbage]

Description

Checks will fail on this PR because commits to master have broken node 14 support since Sept. This does NOT resolve that issue, it simply and blindly steps forward into a 16+ world where things still work.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4a5241e) 100.00% compared to head (7e3ec4f) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #750 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 56 56 Lines 3232 3232 Branches 611 611 ========================================= Hits 3232 3232 ```

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