Correct @since values for wpdb properties

[SergeyBiryukov]

$num_rows	existed in 0.71, declared as var in [13314] (3.0)
$last_query	existed in 0.71, declared as var in [1180] (1.2)
$last_result	existed in 0.71, declared as var in [13314] (3.0)
$col_info	existed in 0.71, declared as var in [1180] (1.2)
$prefix	introduced in [6300] (2.5.0)
$ready	introduced in [6459] (2.3.2)
$dbpassword	introduced in [15537] (3.1), declared as protected in [21473] (3.5)
$dbname	introduced in [15537] (3.1), declared as protected in [21473] (3.5)
$dbhost	introduced in [15537] (3.1), declared as protected in [21473] (3.5)
$dbh	existed in 0.71, declared as protected in [21473] (3.5)

[SergeyBiryukov]

21582.2.patch​ also updates $last_query description, which appears to be inaccurate.

[SergeyBiryukov]

Should @since reflect when a property first appeared, or when it was declared?
There's currently some inconsistency, but I've assumed the former.

[ryan]

In [21521]:

Update @since for various wpdb properties. Props SergeyBiryukov. fixes #21582