niv candid: update 331217ba -> 6961355c

## Changelog for candid:
Branch: master
Commits: [dfinity/candid@331217ba...6961355c](dfinity/candid@331217b...6961355)

* [`a71eb20a`](dfinity/candid@a71eb20) limit size of vec null/reserved ([dfinity/candid⁠#434](https://togithub.com/dfinity/candid/issues/434))
* [`de8b57a0`](dfinity/candid@de8b57a) [Rust] Add composite_query ([dfinity/candid⁠#435](https://togithub.com/dfinity/candid/issues/435))
* [`e212e096`](dfinity/candid@e212e09) add play.motoko.org to Candid UI ([dfinity/candid⁠#437](https://togithub.com/dfinity/candid/issues/437))
* [`c9c85d0a`](dfinity/candid@c9c85d0) bump dependencies for candid UI ([dfinity/candid⁠#442](https://togithub.com/dfinity/candid/issues/442))
* [`e73a8c9f`](dfinity/candid@e73a8c9) fix spec for optional terminal semicolons ([dfinity/candid⁠#445](https://togithub.com/dfinity/candid/issues/445))
* [`f1d573cb`](dfinity/candid@f1d573c) sort define_service ([dfinity/candid⁠#441](https://togithub.com/dfinity/candid/issues/441))
* [`6961355c`](dfinity/candid@6961355) [Rust] add quote for reserved field names in debug trait ([dfinity/candid⁠#446](https://togithub.com/dfinity/candid/issues/446))

nix/sources.json

@@ -6,10 +6,10 @@
         "homepage": "",
         "owner": "dfinity",
         "repo": "candid",
-        "rev": "331217bae379cbebfa531a140f2186c99fae1425",
-        "sha256": "095w2a4lxy2gd7vfjxn7jszm4x3srw8xlxb1zzd096y6h047rxlj",
+        "rev": "6961355c7a8990eee85a667337f7bf664e71f43a",
+        "sha256": "1j1lnb6kjqmaf1z4qkrvyparxrgs1kvvflkbv3q1sblvnz8bqv37",
         "type": "tarball",
-        "url": "https://github.com/dfinity/candid/archive/331217bae379cbebfa531a140f2186c99fae1425.tar.gz",
+        "url": "https://github.com/dfinity/candid/archive/6961355c7a8990eee85a667337f7bf664e71f43a.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "esm": {