github: try without grouped cargo updates

I noticed some cargo dependencies aren't caught by the dependabot. For example,
there are gix updates, but the dependabot somehow thinks it's not possible to
update.

```
updater | 2023/12/14 15:57:52 INFO <job_762380319> Checking if gix 0.55.2 needs updating
  proxy | 2023/12/14 15:57:52 [063] GET https://crates.io:443/api/v1/crates/gix
  proxy | 2023/12/14 15:57:52 [063] 200 https://crates.io:443/api/v1/crates/gix
updater | 2023/12/14 15:57:53 INFO <job_762380319> Latest version is 0.56.0
...
updater | 2023/12/14 15:58:00 INFO <job_762380319> Requirements to unlock update_not_possible
updater | 2023/12/14 15:58:00 INFO <job_762380319> Requirements update strategy bump_versions
updater | 2023/12/14 15:58:00 INFO <job_762380319> No update possible for gix 0.55.2
```

I don't know what's wrong, but let's try without the grouped updates as it was
working before.

FWIW, this issue looks similar:
dependabot/dependabot-core#7896

.github/dependabot.yml

@@ -7,10 +7,6 @@ updates:
   open-pull-requests-limit: 10
   commit-message:
     prefix: "cargo:"
-  groups:
-    cargo-dependencies:
-      patterns:
-      - "*"
 - package-ecosystem: "github-actions"
   directory: "/"
   schedule: