Update workspace-optimizer version in README to 0.12.11

[mikedotexe]

I came to build a cw1 contract and noticed we can update the workspace-optimizer version to the latest here:
https://hub.docker.com/r/cosmwasm/workspace-optimizer/tags

I've also run this locally myself and confirm it works as expected.

[mikedotexe]

Looks as if CI is failing for one test with the result:

Caused by:
  package `base64ct v1.6.0` cannot be built because it requires rustc 1.60 or newer, while the currently active rustc version is 1.59.0

Seems to indicate a need to update the rustc version there

[ueco-jb]

LGTM, thank you.