GDB 18 release schedule proposal

It's time to start thinking about the GDB 18.x release cycle.

After many years, Joel is handing off release manager responsibilities to
Simon and me (Andrew). Joel has kindly agreed to help us with this first
release, but please have patience while we learn the process.

The proposed branch date for GDB 18 is: August 6th.

The target release date will be no earlier than: September 14th. As
always, this release date will depend on any outstanding issues being
resolved.

Let me know if there are any issues with the proposed schedule, or if
there are any GDB issues which should be resolved before we branch.

We are keen that all three of us be around for each stage of this
release in order to learn the process. As a result, the dates given
above could slip if there are any scheduling conflicts. I'll try to give
as much notice as possible if this happens.

Thanks,
Andrew