Where can I find information about past versions of iOS and code-signing windows?

Solution 1:

Currently, no, your only option is to upgrade to iOS 7. This is because Apple is no longer signing iOS 6 and the firmware signing status for iPad 2 shows iOS 7 as the only signed version.

Note: Lookup the signing status (or get the direct download links) for all iOS versions here.

Solution 2:

You probably want to check out this website, which has all past iOS updates. Scroll down and click the iPad 2 model you have (Wifi, GSM, CDMA). Then, simply choose the iOS you want (probably 6.1.3). It will download an .ipsw file.

Then, in iTunes, plug in your iPad. Now, here's the important part: Press Option/Shift while clicking the Check for Update button. A dialog box should open up, and you should navigate to the .ipsw file and then click it. Similarly, if, for some reason, you want to restore your iPad and update, press Option/Shift while clicking the Restore button.