Dependabot: Fix the 'exclude_patterns' config typo

[eskultety]

s/exclude_patterns/exclude-patterns [1].

Since an official native validation solution doesn't exist yet [2] that would work reliably, @bugron/validate-dependabot-yaml was used to validate this typo fix.

fixes: 24f35ee

[1] https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups
[2] dependabot/dependabot-core#4605
[3] https://www.npmjs.com/package/@bugron/validate-dependabot-yaml

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