Firedrake container image for testing is outdated

https://github.com/dolfin-adjoint/pyadjoint/blob/a3ec0aafcfb7048be41917bd9832473f247fbdb6/.circleci/config.yml#L9

I assume this should be `firedrakeproject/firedrake-vanilla-default:dev-main`? I guess somebody with high access than me should do this PR so the CI will actually obey the change...