T15006: Update .arcconfig to point to we.phorge.it

Summary:
Update the `.arcconfig` file to point to our decided-upon URL for hosting

Refs T15006

Test Plan:
I used `arc which` and verified it identified `https://we.phorge.it`:
```lang=console
REPOSITORY
To identify the repository associated with this working copy, arc followed this process:

    Configuration value "repository.callsign" is empty.

    This repository has no VCS UUID (this is normal for git/hg).

    The remote URI for this working copy is
    "ssh://git@we.phorge.it/source/phorge.git".

    Found a unique matching repository.

    This working copy is associated with the Phorge repository.
```

Reviewers: avivey, chris, tobiaswiese

Reviewed By: avivey, tobiaswiese

Maniphest Tasks: T15006

Differential Revision: https://we.phorge.it/D25001

.arcconfig

@@ -1,5 +1,5 @@
 {
-  "phabricator.uri": "https://secure.phabricator.com/",
+  "phabricator.uri": "https://we.phorge.it/",
   "load": ["src/"],
   "history.immutable": false
 }