Getting Duplicate entry issue in dag_tag

### Apache Airflow version

Other Airflow 2 version (please specify below)

### If "Other Airflow 2 version" selected, which one?

2.5.0

### What happened?

When updating the tags for a dag for upper-case to lower-case the scheduler give this info and the Dag does not appear on the UI.

`sqlalchemy.exc.PendingRollbackError: This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original exception was: (MySQLdb._exceptions.IntegrityError) (1062, "Duplicate entry 'pacman-Campaign-711_PRD-1671' for key 'dag_tag.PRIMARY'")
[parameters: (('pacman', 'Campaign-711_PRD-1671'), ('pu', 'Campaign-711_PRD-1671'))] `

### What you think should happen instead?

There should be no such error if the Dag tag is updated

### How to reproduce

Im using MYSQL Server version: 8.0.28 Source distribution

``` CREATE TABLE `dag_tag` (
`name` varchar(100) NOT NULL,
`dag_id` varchar(250) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL,
PRIMARY KEY (`name`,`dag_id`),
KEY `dag_tag_dag_id_fkey` (`dag_id`),
CONSTRAINT `dag_tag_dag_id_fkey` FOREIGN KEY (`dag_id`) REFERENCES `dag` (`dag_id`) ON DELETE CASCADE ON UPDATE RESTRICT
) ENGINE=InnoDB DEFAULT CHARSET=latin1```

Update the tags of a dag from upper-case to lower-case.

### Operating System

centos 7

### Versions of Apache Airflow Providers

_No response_

### Deployment

Official Apache Airflow Helm Chart

### Deployment details

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by tracing the scheduler's DAG-tag update path and the dag_tag primary-key behavior when a tag changes case. Reproduce the update against MySQL 8.0.28 using the schema and steps in the issue, then verify that the transaction completes without a duplicate-entry error and that the DAG remains visible in the UI.

Written by the indexing model from the issue text.