services/mattermost: bump to 6.1.2 (security update)

cf. https://mattermost.com/blog/mattermost-security-updates-6-3-1-6-2-2-6-1-2-5-37-7-released/

this supposedly fixes a "medium-level security vulnerability", but
they're not telling us what it is (for now …) :rolls_eyes:

anyways, seems to run fine on parsons.

nix/sources.json

@@ -23,19 +23,19 @@
         "homepage": "https://mattermost.com",
         "owner": "mattermost",
         "repo": "mattermost-server",
-        "rev": "aa0b717af7e1325d178537d16fa4b50d3f21edae",
+        "rev": "3172adfce9d98fe8f9c98ccd0a0fdbb52291ae0a",
-        "sha256": "0ni10mv38qjyyrygs66ja3ayhn61rhbxxvvfr3rmkkn5mvpzyp92",
+        "sha256": "1sy0kydp87pwby0whgq678jq1zpivqndip81787r9b3dqcyq47cp",
         "type": "tarball",
-        "url": "https://github.com/mattermost/mattermost-server/archive/refs/tags/v6.1.0.tar.gz",
+        "url": "https://github.com/mattermost/mattermost-server/archive/refs/tags/v6.1.2.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/refs/tags/v<version>.tar.gz",
-        "version": "6.1.0"
+        "version": "6.1.2"
     },
     "mattermost-webapp": {
-        "sha256": "1g47qgy9fg4jzgywpsbzkvjk24llgxyfqlzf1ghsaqgcwjvnm594",
+        "sha256": "1m337g6yqjmfk1n39l8sx83hrz0fzipwkqvnkwj7nrs7j3yhndw0",
         "type": "tarball",
-        "url": "https://releases.mattermost.com/6.1.0/mattermost-6.1.0-linux-amd64.tar.gz",
+        "url": "https://releases.mattermost.com/6.1.2/mattermost-6.1.2-linux-amd64.tar.gz",
         "url_template": "https://releases.mattermost.com/<version>/mattermost-<version>-linux-amd64.tar.gz",
-        "version": "6.1.0"
+        "version": "6.1.2"
     },
     "niv": {
         "branch": "master",