Deprecate `docker.attachShellCommand.*` settings

[bwateratmsft]

In order to unblock #251, we need to remove two settings:

• docker.attachShellCommand.linuxContainer
• docker.attachShellCommand.windowsContainer

Usage of these settings is very low. As a workaround when these are removed, the user can set anything for an attach command via command customization.

We can deprecate these for a release, and put in a release note; and then remove them.

• Note: Part of the function of docker.attachShellCommand.linuxContainer is to pick between bash and sh, depending on what's present. With a very simple docker exec command ahead of time, we can determine whether to run bash or sh, and then proceed with executing it, rather than trying to deal with the shell quoting woes.

[bwateratmsft]

Adding label "fixed in next release" so we remember to put this into the changelog.