There'll be some that don't need to use manifest_version 2, and can be easily downgraded...
Some might not be so easy
Simple modifications (removing value):
* PagedTiffHandler
More extensive modifications (moving things to onRegistration):
* TorBlock
* TrustedXff
No action:
* ZeroBanner
* ZeroPortal
----
```
reedy@ubuntu64-web-esxi:/var/www/wiki/mediawiki/extensions$ grep -R manifest_version */extension.json | grep -v 1
BlockAndNuke/extension.json: "manifest_version": 2
GeoData/extension.json: "manifest_version": 2
GlobalBlocking/extension.json: "manifest_version": 2
JsonConfig/extension.json: "manifest_version": 2
LiquidThreads/extension.json: "manifest_version": 2
OAuth/extension.json: "manifest_version": 2
PagedTiffHandler/extension.json: "manifest_version": 2
PageForms/extension.json: "manifest_version": 2
PageImages/extension.json: "manifest_version": 2
SemanticForms/extension.json: "manifest_version": 2
ThrottleOverride/extension.json: "manifest_version": 2
timeline/extension.json: "manifest_version": 2
TorBlock/extension.json: "manifest_version": 2
TrustedXFF/extension.json: "manifest_version": 2
ZeroBanner/extension.json: "manifest_version": 2
ZeroPortal/extension.json: "manifest_version": 2
```