Switch the default `stacks` list to the wildcard stack

[edmorley]

Currently if the URL doesn't specify a stacks list via the stacks parameter, cnb-shim uses a default value like: https://github.com/heroku/cnb-shim/blob/81f6a88f9678ece00b42c1994cfd299cca3851ee/server/main.go#L85

This default value is:

• somewhat of a guess, since there is no way that cnb-shim can know what stacks the buildpack is actually compatible with
• then something we have to continually keep up to date (eg #42 and #72)

We should consider switching it to the wildcard/any stack, *, which permits all stacks.

However, the any stack feature requires Buildpack API 0.5+: https://github.com/buildpacks/spec/releases/tag/buildpack%2Fv0.5

...but cnb-shim is still using Buildpack API 0.4, so switching the default stacks list is blocked on #69.

[edmorley]

Wontfix is cnb-shim is EOL.