Strawman implementation:

• Before we ship a release, guess when we will EOL it (this is generally known, but in theory we can change the EOL so its not perfect)
• In istioctl. add if today > EOL date { warn }
• In helm, NOTES.txt can do the same thing

The reason we guess the EOL is because we don't want to do a dynamic internet lookup. Because we might be wrong, we may want to say something like "Warning: may be installing an EOL version: see istio.io/support for supported releases" or something, rather than "You are installing a EOL version"

cc @therealmitchconnors