feat(logs): additional log retention periods (#20347)

Adds newly supported log retention periods per [the docs](https://docs.aws.amazon.com/AmazonCloudWatchLogs/latest/APIReference/API_PutRetentionPolicy.html#API_PutRetentionPolicy_RequestSyntax). Resolves #20346

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### Adding new Unconventional Dependencies:

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### New Features

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Author: bcelenza

packages/@aws-cdk/aws-logs/lib/log-group.ts

@@ -313,6 +313,26 @@ export enum RetentionDays {
    */
   FIVE_YEARS = 1827,
 
+  /**
+   * 6 years
+   */
+  SIX_YEARS = 2192,
+
+  /**
+   * 7 years
+   */
+  SEVEN_YEARS = 2557,
+
+  /**
+   * 8 years
+   */
+  EIGHT_YEARS = 2922,
+
+  /**
+   * 9 years
+   */
+  NINE_YEARS = 3288,
+
   /**
    * 10 years
    */