doc: release_cycle: Use variable substitution for next version

doc: release_cycle: Use variable substitution for next version

To avoid the problem fixed in commit 57a95d522ca8 ("doc: release_cycle:
fix next release version") moving forward, make use of the variable
substitution feature of rST. This adds a next_ver variable and
references it in all of the places where I had been listing the version
being worked on.

Suggested-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Tested-by: Quentin Schulz <quentin.schulz@cherry.de>