osxkeychain: replace deprecated SecKeychain API

osxkeychain: replace deprecated SecKeychain API

The SecKeychain API was deprecated in macOS 10.10, nearly 10 years ago.
The replacement SecItem API however is available as far back as macOS
10.6.

While supporting older macOS was perhaps prevously a concern,
git-credential-osxkeychain already requires a minimum of macOS 10.7
since 5747c8072b (contrib/credential: avoid fixed-size buffer in
osxkeychain, 2023-05-01) so using the newer API should not regress the
range of macOS versions supported.

Adapting to use the newer SecItem API also happens to fix two test
failures in osxkeychain:

    8 - helper (osxkeychain) overwrites on store
    9 - helper (osxkeychain) can forget host

The new API is compatible with credentials saved with the older API.

Signed-off-by: Bo Anderson <mail@boanderson.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>