INSTALL: mention that we need libcurl 7.19.4 or newer to build

Without NO_CURL=Y we require at least version "7.19.4" of libcurl, see
644de29e22 (http: drop support for curl < 7.19.4, 2021-07-30). Let's
document this in the "INSTALL" document.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

INSTALL

@@ -144,6 +144,10 @@ Issues of note:
 	  not need that functionality, use NO_CURL to build without
 	  it.
 
+	  Git requires version "7.19.4" or later of "libcurl" to build
+	  without NO_CURL. This version requirement may be bumped in
+	  the future.
+
 	- "expat" library; git-http-push uses it for remote lock
 	  management over DAV.  Similar to "curl" above, this is optional
 	  (with NO_EXPAT).