replace planner-version with planner_version in vtexplain

[systay]

Description

vtexplain mistakenly used planner-version instead of planner_version, note the underscore vs the hyphen.

We want to keep the same flag that the vtgate is using, so this PR changes it to use the new flag.

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

[github-actions]

Review Checklist

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

General

• Ensure that the Pull Request has the correct release notes label. release notes none should only be used for PRs that are so trivial that they need not be 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.

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.

[frouioui]

On the doc, this page has to change as well

[systay]

This PR pairs very well together with vitessio/website#1039

[deepthi]

I believe the new convention for flags is - not _. So it might be better do make the change in the opposite direction - i.e. introduce planner-version in vtgate and allow either/or, and revert the change to vtexplain.
@ajm188 can you confirm?

[ajm188]

++ please use - when adding new flags; it'll make my life easier in v15 ❤️