deps: update npm in LTS to 2.15.11

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

• deps

Description of change

Ahoy! It's been a while since we sent over an LTS release, but here we go. This PR includes two releases. They're mostly small, documentation changes, but there's a couple of things that are fairly notable:

• 8f71038 npm/npm#13892 Update LICENSE file to match license on master. (@rvagg)
• npm/npm#13023 npm/npm#13023 npm/npm#12669 Fixes a couple of Windows corner cases and some cache races.
• Smaller bugfixes
• Documentation updates

Changelogs

• v2.15.10
• v2.15.11

r @rvagg r @TheAlphaNerd r @Fishrock123