Remove MariaDB 10.2 Unit Test in v15

[mattlord]

Description

MariaDB 10.2 is now EOL:

• https://mariadb.com/kb/en/mariadb-server-release-dates/
• https://endoflife.date/mariadb

And we deprecated MariaDB support in v14+:

• RFC: Deprecate MariaDB Support in 2022 and Remove in 2023 #9518

We will keep the 10.3 unit test indefinitely. And we will have to think about what/how to test things long term as we always want to support a migration pathway from any MariaDB version into Vitess <latest> (see follow-up ticket).

Related Issue(s)

• Follow-up to: RFC: Deprecate MariaDB Support in 2022 and Remove in 2023 #9518
• Follow-up: Add e2e VReplication Test from MariaDB to MySQL #10732

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a new flag is being introduced, review whether it is really needed. The flag names should be clear and intuitive (as far as possible), and the flag's help should be descriptive. Additionally, flag names should use dashes (-) as word separators rather than underscores (_).
• If a workflow is added or modified, each items in Jobs should be named in order to mark it as required. If the workflow should be required, the GitHub Admin should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should either include a link to an issue that describes the bug OR an actual description of the bug and how to reproduce, along with a description of the fix.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.