Fix Matrix Room Topic Update (opencast#6827)

Our current Matrix roo topic updates are broken.
The reason seems to be the API complaining about the HTTP call we do:

```json
{
  "errcode":"M_NOT_JSON",
  "error":"Invalid Content-Type header: expected application/json"
}
```

This should hopefully fix the problem. It also finally switches from
using the actual username and password to just using a pre-generated
token.


### Your pull request should…

* [x] have a concise title
* [x] [close an accompanying
issue](https://docs.opencast.org/develop/developer/#participate/development-process/#automatically-closing-issues-when-a-pr-is-merged)
if one exists
* [x] [be against the correct
branch](https://docs.opencast.org/develop/developer/development-process#acceptance-criteria-for-patches-in-different-versions)
* [x] include migration scripts and documentation, if appropriate
* [x] pass automated tests
* [x] have a clean commit history
* [x] [have proper commit messages (title and body) for all
commits](https://medium.com/@steveamaza/e028865e5791)
* [x] explain why it needs to be merged into the legacy branch, if it is
targeting the legacy branch

Author: lkiesow

.github/workflows/update-matrix-room.yml

@@ -1,4 +1,5 @@
 on:
+  workflow_dispatch:
   push:
     tags:
       - '*.*'
@@ -11,17 +12,13 @@ jobs:
     steps:
       - name: update room description
         env:
-          MATRIX_PASS: ${{ secrets.MATRIX_BOT_PASS }}
           MATRIX_USER: ${{ secrets.MATRIX_BOT_USER }}
-          URL_LOGIN: 'https://matrix.org/_matrix/client/r0/login'
-          URL_LOGOUT: 'https://matrix.org/_matrix/client/r0/logout'
+          MATRIX_TOKEN: ${{ secrets.MATRIX_BOT_TOKEN }}
           URL_TOPIC: 'https://matrix-client.matrix.org/_matrix/client/r0/rooms/!HBsLKoOIEEOkxAeQvC%3Amatrix.org/state/m.room.topic/'
         run: |
           TAGS="$(curl https://api.github.com/repos/opencast/opencast/tags | jq -r '.[].name')"
           MAJOR="$(echo "$TAGS" | sed 's/\..*$//' | sort --version-sort | tail -n1)"
           STABLE="$(echo "$TAGS" | sort --version-sort | tail -n1)"
           LEGACY="$(echo "$TAGS" | grep -v "^${MAJOR}" | sort --version-sort | tail -n1)"
           TOPIC='{"topic":"opencast.org\n–\nLatest releases: '"$STABLE, $LEGACY"'\n–\nJoin the technical meeting: Tuesday, 3:15 pm UTC at meet.opencast.video (p: welcome)"}'
-          TOKEN="$(curl -s -XPOST -d '{"type":"m.login.password", "user":"'"$MATRIX_USER"'", "password":"'"$MATRIX_PASS"'", "initial_device_display_name":"GitHub Actions"}' "$URL_LOGIN" | jq -r .access_token)"
-          curl -s -XPUT -d "${TOPIC}" "${URL_TOPIC}?access_token=${TOKEN}"
-          curl -s -XPOST "${URL_LOGOUT}?access_token=${TOKEN}"
+          curl -s --fail -XPUT -H 'Content-Type: application/json' -d "${TOPIC}" "${URL_TOPIC}?access_token=${MATRIX_TOKEN}"