Add 'license-name' to $wgExtensionCredits

Set to value 'GPL-2.0+' per https://spdx.org/licenses/ as suggested
in DefaultSettings.php's documentation for this new setting.

Change-Id: Ie9c73d3e76ced1790d885aa825e238d503e46cd5
Merge "Add 'license-name' to $wgExtensionCredits"