ci: disable scheduled Dependabot version PRs

[AbubakarMahmood]

Summary

• set the NuGet version-update PR limit to zero
• set the GitHub Actions version-update PR limit to zero
• remove now-dead grouping and ignore rules

Why

After the supported SemVer guard landed, Dependabot immediately created five isolated maintenance PRs (#55-#59) and remote branches. That behavior is correct but conflicts with this repository's deliberately tidy, trigger-gated maintenance state.

GitHub documents open-pull-requests-limit: 0 as disabling version-update PRs for an ecosystem. Dependabot security updates use the separate security-update path and remain eligible.

Validation

• parsed .github/dependabot.yml with PyYAML
• asserted both configured ecosystems have a zero version-PR limit
• asserted dead groups and ignore policy is absent
• git diff --check

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