new release key for 2.7 and later

This commit removes the public key named "LinuxCNC 2.6 Release Manager"
and adds a new public key named just "LinuxCNC Release Manager".
I'll use the new key to sign future releases in 2.7 and later.

The new key has the email address <emc-developers@lists.sourceforge.net>,
instead of <seb@highlab.com>.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
LinuxCNC 2.7.0~pre2

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>