Remove instructions for CF CLI v7

[Samze]

CF CLI v7 is out of support so we should remove references from the README that refer to download/usage of it.

[norman-abramovitz]

I disagree that CF CLI v7 should be removed. You would be surprised how many people have the mentality that if it's not broken, do not touch it.

Even after the end of life, there will be people who still need time to get the migration done.