INSTALL: bump libcurl version to 7.21.3

Our documentation claims we support curl versions back to 7.19.5. But we
can no longer compile with that version since adding an unconditional
use of CURLOPT_RESOLVE in 511cfd3bff (http: add custom hostname to IP
address resolutions, 2022-05-16). That feature wasn't added to libcurl
until 7.21.3.

We could add #ifdefs to make this work back to 7.19.5. But given that
nobody noticed the compilation failure in the intervening two years, it
makes more sense to bump the version in the documentation to 7.21.3
(which is itself over 13 years old).

We could perhaps go forward even more (which would let us drop some
cruft from git-curl-compat.h), but this should be an obviously safe
jump, and we can move forward later.

Note that user-visible syntax for CURLOPT_RESOLVE has grown new features
in subsequent curl versions. Our documentation mentions "+" and "-"
entries, which require more recent versions than 7.21.3. We could
perhaps clarify that in our docs, but it's probably not worth cluttering
them with restrictions of ancient curl versions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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