doc: make consistent release notes titles

For a long time, release notes were titled "Zephyr Kernel x.x.x".  With
the 2.0 release the title was changed to "Zephyr RTOS 2.0.0" and for the
1.14.1 update the title was "Zephyr 1.14.1" and for the 2.1.0 working
draft the title went back to "Zephyr Kernel 2.1.0". The end result was
the release notes index looking like this:

   Zephyr Kernel 2.1.0 (Working Draft)
   Zephyr RTOS 2.0.0
   Zephyr 1.14.1
   Zephyr Kernel 1.14.0
   Zephyr Kernel 1.13.0
   Zephyr Kernel 1.12.0
   Zephyr Kernel 1.11.0

I think the intention was to drop the "Kernel" name (since the release
is more than a kernel), so let's make the release notes titles
consistent by calling post 1.14.0 release notes "Zephyr x.x.x"

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>