Issue:
Bug: fix(users): Only use lowercase letters in emails



Currently uppercase letters in emails are being using at it is. Ideally we should convert them to lowercase before performing any DB operations with it.

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR changes will convert emails from request to lowercase before performing any DB operations.

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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Closes #6600.

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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```
curl --location 'http://localhost:8080/user/connect_account' \
--header 'Content-Type: application/json' \
--data-raw '{
    "email": "USER@Example.com"
}'
```

This email in the about request will be inserted as `user@example.com` in DB.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

```
curl --location 'http://localhost:8080/user/connect_account' \
--header 'Content-Type: application/json' \
--data-raw '{
    "email": "USER@Example.com"
}'
```

This email in the about request will be inserted as `user@example.com` in DB.

Patch:
diff --git a/crates/router/src/types/domain/user.rs b/crates/router/src/types/domain/user.rs
index 5a881728b07..4cb69e68ed8 100644
--- a/crates/router/src/types/domain/user.rs
+++ b/crates/router/src/types/domain/user.rs
@@ -103,7 +103,7 @@ impl UserEmail {
     pub fn new(email: Secret<String, pii::EmailStrategy>) -> UserResult<Self> {
         use validator::ValidateEmail;
 
-        let email_string = email.expose();
+        let email_string = email.expose().to_lowercase();
         let email =
             pii::Email::from_str(&email_string).change_context(UserErrors::EmailParsingError)?;
 
@@ -123,21 +123,8 @@ impl UserEmail {
     }
 
     pub fn from_pii_email(email: pii::Email) -> UserResult<Self> {
-        use validator::ValidateEmail;
-
-        let email_string = email.peek();
-        if email_string.validate_email() {
-            let (_username, domain) = match email_string.split_once('@') {
-                Some((u, d)) => (u, d),
-                None => return Err(UserErrors::EmailParsingError.into()),
-            };
-            if BLOCKED_EMAIL.contains(domain) {
-                return Err(UserErrors::InvalidEmailError.into());
-            }
-            Ok(Self(email))
-        } else {
-            Err(UserErrors::EmailParsingError.into())
-        }
+        let email_string = email.expose().map(|inner| inner.to_lowercase());
+        Self::new(email_string)
     }
 
     pub fn into_inner(self) -> pii::Email {