MDEV-34900: Fix stage names for profiling

[SaitejSamudrala]

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• [x] The Jira issue number for this PR is: MDEV-34900

Description

This PR addresses MDEV-34900 by renaming stage_executing to "Prepare for executing" and stage_sending_data to "executing" in JOIN::exec_inner() to better reflect the actual process during profiling.

Release Notes

Renaming few of the profiler stages.

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• [ ] This is a new feature or a refactoring, and the PR is based against the main branch.
• [ ] 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

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