fix bug: should throw err when prepare multi stmt (#14112)

[ouyuanning]

参看这个comment：
#14068 (comment)

Approved by: @qingxinhome

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #14068

What this PR does / why we need it:

参看这个comment：
#14068 (comment)

[matrix-meow]

@ouyuanning Thanks for your contributions!

Title: fix bug: should throw err when prepare multi stmt (#14112)

Body: This pull request fixes a bug related to preparing multiple statements. It is based on the comment provided in issue #14068.

Changes:

• In mysql_cmd_executor.go, the doComQuery function has been modified to check if the command is COM_STMT_PREPARE and there are multiple statements. If so, it throws an error using moerr.NewNotSupported.
• In the handleSetOption function, the code related to CLIENT_MULTI_STATEMENTS has been commented out.

Review:

1. In the doComQuery function, the check for COM_STMT_PREPARE and multiple statements is necessary to ensure that only a single statement is prepared. However, the error message provided in moerr.NewNotSupported is not clear. It would be helpful to provide a more descriptive error message explaining why preparing multiple statements is not supported.

2. In the handleSetOption function, the code related to CLIENT_MULTI_STATEMENTS has been commented out. It is unclear why this change was made and if it is necessary. If it is not necessary, the commented code should be removed to avoid confusion.

Suggestions:

1. In the doComQuery function, update the error message in moerr.NewNotSupported to provide a clear explanation of why preparing multiple statements is not supported. For example, "Preparing multiple statements is not supported. Please prepare one statement at a time."

2. In the handleSetOption function, if the commented code is not necessary, it should be removed to avoid confusion. If it is necessary, add a comment explaining why it is commented out and provide a clear explanation of the implications of this change.

Optimizations:

No optimizations are needed for this pull request.