Add missing `@since` notes to `gallery_shortcode` documentation`

[desrosj]

The gallery_shortcode() function was introduced in version 2.5. It looks like there are some @since tags that could be added for the gallery_shortcode() function to improve the historical documentation.

A few I identified looking through the version control blame trying to track something else down:

• In [27302] the HTML elements used for the itemtag, icontag and captiontag attributes were changed when HTML5 support was added through add_theme_support().
• #24979/[25665] introduced a link attribute in 3.7.
• #6368/[7496] switched to actually using the $attr parameter to shape the output of the shortcode, introduced several new attributes in including size, itemtag, icontag, captiontag, and columns, and also changed the markup from <div> tags to <dl>, <dt> and <dd> tags, etc. for 2.8.

[desrosj]

A good function to use as an example is add_theme_support() (source), which has over a dozen @since tags for historical context.

Adds @since for all major functionality and versions for the gallery_shortcode() functionality.

Trac ticket: https://core.trac.wordpress.org/ticket/56206

[mukesh27]

PR look good to me.

Set 6.1 milestone for visibility

[audrasjb]

I spotted a few small issues in the proposed PR. Adding another commit to fix them.

[ben.greeley]

@audrasjb Thank you for making those edits. I've uploaded a patch with the changes included.

[ben.greeley]

Newest patch with changes

[audrasjb]

In 54172:

Docs: Add missing @since history in Gallery Shortcode function.

Props bengreeley, audrasjb, desrosj, mukesh27.
Fixes #56206.
See #55646.

Committed in https://core.trac.wordpress.org/changeset/54172