http: correct version check for CURL_HTTP_VERSION_2

http: correct version check for CURL_HTTP_VERSION_2

In d73019feb44 (http: add support selecting http version, 2018-11-08)
a dependency was added on CURL_HTTP_VERSION_2, but this feature was
introduced in curl version 7.43.0, not 7.47.0, as the incorrect
version check led us to believe.

As looking through the history of that commit on the mailing list will
reveal[1], the reason for this is that an earlier version of it
depended on CURL_HTTP_VERSION_2TLS, which was introduced in libcurl
7.47.0.

But the version that made it in in d73019feb44 had dropped the
dependency on CURL_HTTP_VERSION_2TLS, but the corresponding version
check was not corrected.

The newest symbol we depend on is CURL_HTTP_VERSION_2. It was added in
7.33.0, but the CURL_HTTP_VERSION_2 alias we used was added in
7.47.0. So we could support an even older version here, but let's just
correct the checked version.

1. https://lore.kernel.org/git/pull.69.git.gitgitgadget@gmail.com/

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