docs: decrease the minimum supported OpenSSL

Our code does not preclude the use of versions 1.0.x of OpenSSL.
Instead, we discourage it's use due to security concerns. Update the
documentation to reflect this.

Change-Id: I5c60907337f10b05d5c43b0384247c5d4135db50
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>