Add dependabot config for GitHub Actions version updates

[Ankitsinghsisodya]

Closes #9034

Summary

• Adds .github/dependabot.yml to enable automatic weekly bumps of GitHub Actions versions
• Addresses the missing automation noted in ci: standardize rtCamp/action-slack-notify to v2.2.1 across workflows #8998 — previously no dependabot config existed for github-actions ecosystem

Test plan

• Verify dependabot opens PRs for outdated actions after merge

[knative-prow]

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[Copilot]

Pull request overview

Adds Dependabot configuration to automate GitHub Actions dependency updates, and aligns Slack notification action versions used in workflows.

Changes:

• Add .github/dependabot.yml enabling weekly updates for the github-actions ecosystem.
• Bump rtCamp/action-slack-notify to v2.3.3 in the weekly Slack reminder workflow.
• Bump rtCamp/action-slack-notify to v2.3.3 in the KinD e2e workflow failure notification.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
.github/dependabot.yml	Introduces Dependabot config to check GitHub Actions dependencies weekly.
.github/workflows/weekly-office-hours-slack-reminder.yaml	Updates Slack notification action version for the scheduled reminder.
.github/workflows/kind-e2e.yaml	Updates Slack notification action version for e2e failure alerts.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.06%. Comparing base (32bc524) to head (3691522).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9033      +/-   ##
==========================================
- Coverage   51.07%   51.06%   -0.01%     
==========================================
  Files         409      409              
  Lines       21999    21997       -2     
==========================================
- Hits        11236    11233       -3     
+ Misses       9903     9901       -2     
- Partials      860      863       +3     

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[creydr]

Can we group those updates into one PR (like here). Also please check on the ignore similar to https://github.com/knative/serving/blob/main/.github/dependabot.yaml

[Ankitsinghsisodya]

Can we group those updates into one PR (like here). Also please check on the ignore similar to https://github.com/knative/serving/blob/main/.github/dependabot.yaml

Done!

[creydr]

/ok-to-test
/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Ankitsinghsisodya, creydr

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Needs approval from an approver in each of these files:

• OWNERS [creydr]

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