fix: correct the nonce argument in the version switch AJAX handlers

[TallblokeUK]

Switching or rolling back the plugin version does nothing. Picking a version and clicking Switch Version shows "An error occurred."; Refresh Available Versions resets its own label without any message at all. Reproduced on a live site as well as locally.

check_ajax_referer( $action, $query_arg ) treats $query_arg as the name of the request key to read the nonce from. Both handlers passed the nonce's value there instead, so WordPress looked for $_REQUEST['<the nonce value>'], fell through to _ajax_nonce and _wpnonce — neither of which the front end sends — and verified an empty string. Every request was rejected, valid nonce or not.

Promotion_Base::dismiss_promotion_ajax_handler() already does this correctly with check_ajax_referer( self::AJAX_ACTION, 'nonce' ), which is the form used here.

Testing

Against admin-ajax.php as a logged-in admin:

	before	after
refresh, valid nonce	403	200 success
refresh, invalid nonce	403	403
switch, valid nonce	403	reaches the handler
switch, invalid nonce	403	403

The switch case was exercised with an empty target_version, so the request passes the nonce and capability checks and stops at validation — proving the gate opens without installing anything.

Driven through the real UI afterwards: the dropdown populates with all 62 versions from wordpress.org and both buttons respond. phpcs clean.

Note this sits behind Settings → Debug → Enable version change, which is off by default, so only people who switched it on will have hit it.

[TallblokeUK]

Superseded by #506, which targets release/v3.10.2/core so it can ship in 3.10.2. Same commit, cherry-picked onto the release branch; retargeting this one directly would have pulled in unrelated core-beta changes to the test files.