Add Node 15 (which includes npm v7) to test matrix

[mikeharder]

Node 15 should be added to our test matrix. In particular, Node 15 includes npm v7 which is a major version bump and includes some big changes.

https://github.blog/2020-10-13-presenting-v7-0-0-of-the-npm-cli/

Since we need to continue testing on Node 8-14, this would be an additional row in our test matrixes, rather than a replacement. However, since we have moved to the 1ES agents, this shouldn't impact our build queue.

Current test matrix: #9462

Child Issues:

• Add Node 15 to CI test matrix #14272
• Add Node 16 to smoke test matrix #14273
• Add Node 15 to live test matrix #14274