Silently failing to load in vim < 9

[teki]

This commit made the plugin silently not load: https://github.com/ervandew/supertab/commit/8c638fe17b470f4a27554cdf8e641bafa5e2a7a4

It is okay to have a minimum documented version, but I just wasted some time to use the plugin on Ubuntu 22, which ships with vim 8.2. Displaying an error message would go a long way to help.

[dandycyclone]

#226 #223 #222 - on the same line.

[iarriagada]

Why does supertab require vim9? The README still talks about vim8+