(typescript/rds/mysq): Don't use "var" keyword

[v-au]

Describe the bug

Parts of the code at typescript/rds/mysql/mysql.ts uses var instead of let which does not follow the Typescript best practices as per AWS Prescriptive Guidance - Best practices for using the AWS CDK in TypeScript to create IaC projects.

Expected Behavior

All declarations use the let keyword.

Current Behavior

There is a mix of let and var.

Reproduction Steps

(main=) typescript $ rg "\b(let|var)\b" rds/mysql/mysql.ts 
106:    var mysqlUsername = "dbadmin";
110:    var ingressSources = [];
114:    var engineVersion = rds.MysqlEngineVersion.VER_8_0_28;
132:    for (let subnetId of props.subnetIds!) {
164:    for (let ingressSource of ingressSources!) {
165:      for (let c of mysqlConnectionPorts) {

Possible Solution

Replace declarations using the var keyword with let.

Additional Information/Context

I will submit a pull request shortly.

CDK CLI Version

2.156.0 (build 2966832)

Framework Version

No response

Node.js Version

v20.17.0

OS

WSL Ubuntu 20.04.6 LTS (Focal Fossa)

Language

Typescript

Language Version

No response

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