Docs: Fix @since tags to use the 3-digit x.x.x style in wp-db.php

[mukesh27]

It was flagged in #26713. As per the ​documentation standards need to change @since 0.71 to @since 0.71.0 in wp-db.php

[mukesh27]

Patch.

[SergeyBiryukov]

Hi @mukesh27, thanks for the patch!

The standards apply to our current ​version numbering scheme, where the first two digits are the major version (e.g. 5.1), and the third one is a minor version (e.g. 5.1.1).

​0.70 and ​0.71 were the very first releases. They didn't use the current verion numbering scheme, so the 3-digit rule does not apply there.

[mukesh27]

@SergeyBiryukov Thanks for your information.