feat: use cross-fetch instead of node-fetch

[jstarry]

Problem

New version of node-fetch no longer includes browser module

Summary of Changes

• Bump min node version to v12.20.0 from v11
• Use cross-fetch instead of using node-fetch directly
• Silence a few new rollup warnings
• Add explicit required node version

Fixes #

[codecov]

Codecov Report

Merging #19621 (b79cf16) into master (bbc4fdb) will decrease coverage by 11.3%.
The diff coverage is 100.0%.

@@             Coverage Diff             @@
##           master   #19621       +/-   ##
===========================================
- Coverage    82.5%    71.1%    -11.4%     
===========================================
  Files         468       34      -434     
  Lines      131676     1995   -129681     
  Branches        0      280      +280     
===========================================
- Hits       108686     1420   -107266     
+ Misses      22990      477    -22513     
- Partials        0       98       +98