Use base64 0.21.0

Bug: 268206364
Ignore-AOSP-First: This change requires a newer version of base64.
                   Will cherry-pick to aosp once this change is merged.
Test: atest
Test: browsed some websites, verified DoH worked with dns.google,
      dns64.dns.google, and cloudflare-dns.com
Change-Id: I83799c36a1028564468b2335940a20659955fe21