v1.21.0 release

Bug Fixes 🐞

• fix: reuse armclient connections to fix performance issues (#1426)
• fix: VMSSList throttling when VM is in Deleting state (#1426)
• fix: CSI function should return internal error (#1418)

What's Changed

• fix: CSI function should return internal error by @andyzhangx in #1418
• chore: upgrade azure cloud provider lib by @andyzhangx in #1426

Full Changelog: v1.20.0...v1.21.0