message: chore: drop Ubuntu Xenial support
  Use Bionic as the least supported Ubuntu version.
diff: @@ -20,7 +20,7 @@ We ensure that these platforms will satisfy the following requirements:
  | OS | Arch | CKB VM Mode |
  | --- | --- | --- |
  -| Ubuntu 16.04 | x64 | ASM |
  +| Ubuntu 18.04 | x64 | ASM |
  | macOS | x64 | ASM |
  The Tier 1 requires CPU to support at least SSE4.2, and AVX is recommended.
  @@ -34,7 +34,6 @@ The official binary releases are also provided for the Tier 2 platforms.
  | OS | Arch | CKB VM Mode |
  | --- | --- | --- |
  | Windows \* | x64 | ASM |
  -| Ubuntu 18.04 | x64 | ASM |
  | Ubuntu 20.04 | x64 | ASM |
  | Debian Stretch | x64 | ASM |
  | Debian Buster | x64 | ASM |
  