Change all "into CoreFX master" subscriptions to new VersionTools build task

[dagood]

The new command looks like:

build-managed.cmd -- /t:UpdateDependencies
  /p:GitHubUser=dotnet-bot
  /p:GitHubEmail=dotnet-bot@microsoft.com
  /p:GitHubAuthToken=...
  /p:ProjectRepoOwner=dotnet
  /p:ProjectRepoName=corefx
  /p:ProjectRepoBranch=master
  /p:NotifyGitHubUsers="dotnet/corefx-contrib"

UpdateDependencies updates all corefx dependencies at the same time (to put it in the same PR) so the same command happens no matter which dependency changes. (Ripe for a refactor into multiple paths per subscription!)

When the VersionTools changes get merged into dev/api I'll need to change the subscription for that branch too.

/cc @weshaggard @eerhardt

[dagood]

Merging this in so I can kick off a build and see if it works end to end.

[weshaggard]

👍

I assume we are still planning on changing the structure of this at some other point to have a list of files to monitor instead of needing to duplicate the update commands.

[dagood]

Yep, I think it's just a matter of prioritization at this point.