Auth: Timestamp in signin email is formatted confusingly, is not localized

### Operating System

MacOS 14.2

### Browser Version

Chrome 120.0

### Firebase SDK Version

10.7.1

### Firebase SDK Product:

Auth

### Describe your project's tooling

Firebase JS SDK, Firebase Auth with Email/Password + Email link (passwordless sign-in).

### Describe the problem

First of all, sorry for opening the issue in the JS SDK repo, but it seems a lot of service-related issues are opened here.

The timestamp in the Firebase-generated Email link (passwordless sign-in) email subject introduced in https://github.com/firebase/firebase-js-sdk/issues/2574 has an uncommon format and without any timezone information (e.g. 2024 January 11 16:50 Z) and is the same in all locales, even those [officially supported](https://firebase.google.com/docs/auth/web/email-link-auth#default-email).

I understand that the template for this email can't be customized due to security reasons; however, our customers have expressed concern due to the email looking "shady" or unprofessional since it mixes languages, which is also not uncommon in spam/scam mails.

My expectation would be the timestamp being formatted in a common format (e.g. January 11, 2024, 16:50 UTC) localized in the language of the rest of the email.


### Steps and code to reproduce issue

- Set up the Firebase JS SDK (or any other frontend SDK).
- (optional) Set the SDK language: `getAuth().languageCode = "de";`
- Call `sendSignInLinkToEmail(getAuth(), "test@my-url.com", {url: "https://my-url.com", handleCodeInApp: true});`