Add GitHub action to assign reviewers on PR created by Dependabot

[Torch3333]

Description

#2681 sets up a CODEOWNERS file to automatically assign the ScalarDB team as reviewers to any pull request created.
This was introduced primarily to assign reviewers for PR created by Dependabot.
However, setting up CODEOWNERS also assigns reviewers for the backport PR, which is not wanted.
Instead, we create a GitHub action to assign reviewers only for PRs created by Dependabot.

Related issues and/or PRs

• Set up CODEOWNERS to automatically assign reviewers for new PR #2681

Changes made

• Add action to assign reviewer to PR that Dependabot creates
• Remove the codeowner file

Checklist

The following is a best-effort checklist. If any items in this checklist are not applicable to this PR or are dependent on other, unmerged PRs, please still mark the checkboxes after you have read and understood each item.

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• I have considered whether similar issues could occur in other products, components, or modules if this PR is for bug fixes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

N/A

Release notes

N/A

[komamitsu]

LGTM! 👍

[brfrn169]

LGTM! Thank you!

[feeblefakie]

LGTM! Thank you!