chore: enable Dependabot automatic version upgrade PRs

[ianbotsf]

Issue #

(none)

Description of changes

This change should enable Dependabot to send PRs to us when new versions of Smithy are launched. Only Smithy is allowlisted for now—we can see if it makes sense to enable for other dependencies in the future.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK

[github-actions]

Affected Artifacts

No artifacts changed size

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[ianbotsf]

As noted by @lauzadis this change would stop us from receiving automatic security version bump PRs.

From documentation of the allow parameter:

Use the allow option to customize which dependencies are updated. This applies to both version and security updates.

This leaves us with no way to define separate rules for security updates and non-security version upgrades. Since automated security updates are more important, I'm closing this PR for now until such time as feature requests like dependabot/dependabot-core#6380 or dependabot/dependabot-core#1778 are addressed.