[Breaking Change] Action API: removal of "extension" tag source in ApiQueryTags

Hello,
In MediaWiki 1.42, within the list=tag API query module, the tag source 
type named "extension" was renamed to "software" [1]. The "extension" 
entries were made to still
appear alongside "software" in the tag source list but were deprecated 
[2][3]. There are plans to remove this compatibility behavior in the 
next major version of MediaWiki (1.47), so that the "extension" entries 
will no longer appear.

The use of "extension" is misleading because it does not exclusively 
refer to tags defined by MediaWiki extensions (via the onListDefinedTags 
hook), but also those defined in MediaWiki core (via 
ChangeTagsStore::DEFINED_SOFTWARE_TAGS). The distinction isn't really 
useful to clients anyway.

[1] https://phabricator.wikimedia.org/T247552
[2] 
https://en.wikipedia.org/w/api.php?action=query&list=tags&tgprop=source&tglimit=max
[3] 
https://lists.wikimedia.org/hyperkitty/list/[email protected]/thread/EL3ECIAJWSBBMGDH2SXH6EJOQJWRGKAJ/