MDEV-31751 SET @@tx_isolation=X behaves like SET TRANSACATION ISOLATION LEVEL X instead of SET SESSSION tx_isolation=X

[grooverdan]

• [x] The Jira issue number for this PR is: MDEV-31751

Description

SET @@tx_isolation=X behaves like SET TRANSACATION ISOLATION LEVEL X instead of SET SESSSION tx_isolation=X

But that doesn't mean @@transaction_isolation needs to. Take the opportunity before GA to fix its behaviour of @@transaction_isolation to behave like a session variable.

How can this PR be tested?

Note this is based on top of the #2707 because it updates the MTR tests to show that @@tx_isolation doesn't emit session tracking information (like SET TRANSACTION ... ) but {{@@transaction_isolation}} does.

Basing the PR against the correct MariaDB version

• [ ] This is a new feature and the PR is based against the latest MariaDB development branch.
• [ X ] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [ X ] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [ X ] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

Based on #2707 for Buildbot testing. Will rebase on to 11.1 properly before merge, adjusting the mysqltest_tracking_info.test if needed.

[dr-m]

The words TRANSACTION and SESSION are typed incorrectly with an extra A and S, respectively.