templates: Reinstate unused version comparison functions with warning

templates: Reinstate unused version comparison functions with warning

Revert the commit a79c567f6 (templates: Remove unused version comparison
functions) and add a warning to the functions that they are deprecated.

Removing the functions directly caused a lot of upgrade issues
with custom user scripts that called the functions. In Debian and
Ubuntu, grub-mkconfig is invoked as a post-installation script
and would fail, causing upgrades to fail halfway through and
putting the package manager into an inconsistent state.

FWIW, we get one bug per 2 weeks basically, for an interim Ubuntu
release which generally does not receive much usage, that is a high
number.

The proposal is to pick this for 2.12 and directly after the release
remove it again. Then users will have time to fix their scripts without
systems breaking immediately.

This reverts commit a79c567f6 (templates: Remove unused version
comparison functions).

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Julian Andres Klode <julian.klode@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>