Add ignore filters to dependabot.yml

[fxamacker]

Prevent accidental version bumps to avoid potential compatibility issues for:

• fxamacker/cbor
• fxamacker/circlehash

E.g. We use stream-mode branch of fxamacker/cbor, etc. so we don't want automatic update suggestions like PR #288.

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• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels