Move Dependabot day to Wednesday and ignore major upgrades

[amishas157]

Summary

Two small changes to .github/dependabot.yml:

1. Schedule day moved sunday → wednesday so Dependabot PRs land mid-week and aren't sitting through the weekend.
2. Major-version upgrades ignored via ignore: [{dependency-name: "*", update-types: [version-update:semver-major]}] on every updates entry. Major bumps were repeatedly creating PRs that couldn't pass CI due to transitive constraints (e.g. apache-airflow-providers-cncf-kubernetes 10.16.1 requiring cryptography ≥44 vs gcloud-aio-auth capping at <42; click 8.3.x capped by sqlfluff 3.5.0). Major upgrades will be done manually when the surrounding deps can absorb them.

For the dbt/airflow repos that previously had a separate major group entry, that group is now redundant and removed.

Test plan

• Verify YAML is valid (Dependabot will surface a config error in the repo's Insights → Dependency graph → Dependabot tab if not).

[Copilot]

Pull request overview

Updates the repository’s Dependabot configuration to better align dependency update timing with the work week and to reduce CI-noisy PRs caused by major-version bumps that frequently fail due to transitive constraints.

Changes:

• Moved the weekly Dependabot schedule from Sunday to Wednesday (02:00) for all configured ecosystems.
• Added a global ignore rule to suppress semver-major updates for all dependencies across all ecosystems.
• Kept existing grouping behavior for Go modules, GitHub Actions, and Docker updates.

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