Upgrade Java version for `facia-purger` and `backup-parameter-store` lambdas

They are both currently on Java 11, which will be deprecated on the 30th of June (30/6/26)[^1]. We should upgrade to at least Java 21[^2] by then, but could probably go straight to Java 25. We may want to switch `backup-parameter-store` to CDK at the same time, as mentioned in #72.

https://github.com/guardian/frontend-lambda/blob/b0562a56f615b77fd38d1731b2226cbd4adb083d/cdk/lib/facia-purger.ts#L44

https://github.com/guardian/frontend-lambda/blob/b0562a56f615b77fd38d1731b2226cbd4adb083d/backup-parameter-store/cfn.yaml#L94

[^1]: https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html#runtimes-supported
[^2]: https://github.com/guardian/maintaining-scala-projects/issues/8