Prevent Dependabot version updates from running on HHS:main

[jtwillis92]

Description:
Recently a dependabot.yml file was added to this repo to configure Dependabot to automatically issue pull requests nightly for any dependencies listed in package.json or Pipfile. However, to keep with our current git flow we don't want these pull requests to be issued against the main HHS repository. Automatic security updates have already been disabled on the HHS repo but those work distinctly from version updates where the configuration file is the source of truth and as such those PRs still get opened.

To resolve this we can specify a target_branch in the configuration file so these version updates only check and issue PRs against the raft-tdp-main branch. Since this branch doesn't exist in HHS the PRs will not get opened.

Acceptance Criteria:
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• Dependabot version update PRs do not get opened against HHS:main
• Testing Checklist has been run and all tests pass
• README is updated, if necessary

Tasks:
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• Add a target_branch configuration to the dependabot.yml configuration file
• Run Testing Checklist and confirm all tests pass

Notes:
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• Note about impact of this change

Supporting Documentation:
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Open Questions:
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