INSTALL: document requirement for libcurl 7.61.0

INSTALL: document requirement for libcurl 7.61.0

Our platform support policy states that we require "versions of
dependencies which are generally accepted as stable and supportable,
e.g., in line with the version used by other long-term-support
distributions".  Of Debian, Ubuntu, and RHEL, the three most common
distributions that provide LTS versions, the version with mainstream
long-term security support with the oldest libcurl is 7.61.0 in RHEL 8.

Update the documentation to state that this is the new base version for
libcurl.  Remove text that is no longer applicable to older versions.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>