chore(rds): add MySQL 5.7.12 version

[AkitoAndo]

Issue # (if applicable)

Closes #.

Reason for this change

MySQL 5.7.12 version is missing from the AWS CDK MySQL engine versions. The current CDK library starts from MySQL 5.7.16, leaving 5.7.12 undefined despite AWS RDS supporting this version.

Description of changes

Changes made:

• Added MySQL 5.7.12 version to packages/aws-cdk-lib/aws-rds/lib/instance-engine.ts
• Positioned it appropriately before MySQL 5.7.16 to maintain version ordering
• Added @deprecated annotation consistent with other deprecated 5.7.x versions

Why these changes address the issue:

• AWS RDS supports MySQL 5.7.12, but it's not defined in the CDK library
• Maintains consistency with existing codebase by adding the deprecated marker
• Preserves version ordering by placing it before 5.7.16

Alternatives considered and rejected:

• Considered adding without deprecated marker, but rejected to maintain consistency with all other 5.7.x versions which have deprecated markers

Design decisions:

• Followed existing code pattern using MysqlEngineVersion.of('5.7.12', '5.7') format
• Unified comment style with other version definitions

Describe any new or updated permissions being added

No new IAM permissions are required for this change. Only existing RDS-related permissions are used.

Description of how you validated changes

Validation methods:

1. Code review: Verified consistency with existing MySQL version definitions
2. Syntax check: Confirmed no TypeScript syntax errors
3. Pattern matching: Verified definition follows the same pattern as other deprecated versions

Testing:

• Manually reviewed code to ensure it follows existing patterns
• Verified correct version ordering placement

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: a9407f7
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[mazyu36]

Thank you for the contribution.
Just for clarification, can this enum be used for deployment?
I think 5.7.12 was deprecated long ago, so this enum cannot be used anymore.
Could you please let me know what use cases this change is intended for?

[gasolima]

Thanks for the contribution!

Right now the only my SQL Extended support
is 5.7.44. So the 5.7.12 isn't supported anymore.

Could you please let me know what use cases this change is intended for?

[github-actions]

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