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  Remove text that is no longer applicable to older versions.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>

INSTALL

@@ -129,17 +129,12 @@ Issues of note:
 	  itself, e.g. Digest::MD5, File::Spec, File::Temp, Net::Domain,
 	  Net::SMTP, and Time::HiRes.
 
-	- git-imap-send needs the OpenSSL library to talk IMAP over SSL if
-	  you are using libcurl older than 7.34.0.  Otherwise you can use
-	  NO_OPENSSL without losing git-imap-send.
-
 	- "libcurl" library is used for fetching and pushing
 	  repositories over http:// or https://, as well as by
-	  git-imap-send if the curl version is >= 7.34.0. If you do
-	  not need that functionality, use NO_CURL to build without
-	  it.
+	  git-imap-send. If you do not need that functionality,
+	  use NO_CURL to build without it.
 
-	  Git requires version "7.21.3" or later of "libcurl" to build
+	  Git requires version "7.61.0" or later of "libcurl" to build
 	  without NO_CURL. This version requirement may be bumped in
 	  the future.