chore(appconfig): compilation error in ts 4.2 and below because of using get in interface (#29226)

### Reason for this change

There is a lint error which `get` syntax can't be used on interface before TS version 4.3

### Description of changes

Replace `get` with a regular function.

### Description of how you validated changes

Unit test passed.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: xazhao

packages/aws-cdk-lib/aws-appconfig/lib/application.ts

@@ -72,7 +72,7 @@ export interface IApplication extends cdk.IResource {
   /**
    * Returns the list of associated environments.
    */
-  get environments(): IEnvironment[];
+  environments(): IEnvironment[];
 
   /**
    * Adds an extension defined by the action point and event destination
@@ -204,7 +204,7 @@ abstract class ApplicationBase extends cdk.Resource implements IApplication, IEx
     this._environments.push(environment);
   }
 
-  get environments(): IEnvironment[] {
+  public environments(): IEnvironment[] {
     return this._environments;
   }
 

packages/aws-cdk-lib/aws-appconfig/lib/configuration.ts

@@ -304,7 +304,7 @@ abstract class ConfigurationBase extends Construct implements IConfiguration, IE
 
   protected addExistingEnvironmentsToApplication() {
     this.deployTo?.forEach((environment) => {
-      if (!this.application.environments.includes(environment)) {
+      if (!this.application.environments().includes(environment)) {
         this.application.addExistingEnvironment(environment);
       }
     });
@@ -315,7 +315,7 @@ abstract class ConfigurationBase extends Construct implements IConfiguration, IE
       return;
     }
 
-    this.application.environments.forEach((environment) => {
+    this.application.environments().forEach((environment) => {
       if ((this.deployTo && !this.deployTo.includes(environment))) {
         return;
       }