Dependabot ignores "pull-request-branch-name.separator" option

Dependabot began to ignore `pull-request-branch-name.separator` option from YML settings.

Details:
- Dependabot YML file in my repo was not touched last few months
- newly created Pull Request has the wrong branch name separator
- branch name separator option is configured according to the [documentation](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#insecure-external-code-execution)
Here is my `.github/dependabot.yml` file content:
```
version: 2
updates:
- package-ecosystem: bundler
directory: "/"
schedule:
interval: daily
time: "10:00"
timezone: Europe/Berlin
pull-request-branch-name:
separator: "-"
target-branch: dependency-updates
```

Newly created PR with unexpected branch name: (separated by `/`):

Example of existing PR with expected branch name: (separated by `-`), created one week ago:

Start with the reported `.github/dependabot.yml` configuration and compare the configured `pull-request-branch-name.separator` with the generated branch name. Reproduce the issue using the Bundler ecosystem and verify that newly created pull requests use the configured separator; no source file or test is identified in the report.

Written by the indexing model from the issue text.