MDEV-35837: Update CODING_STANDARDS to C++17

[ParadoxV5]

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• [x] The Jira issue number for this PR is: MDEV-35837
  • #3853 fixup

What problem is the patch trying to solve?

#3853 intended to update CODING_STANDARDS.md to say C++17 re. #3758, but somehow didn’t (lost diff when I squashed?), and it still says C++11.

Release Notes

N/A

How can this PR be tested?

N/A?

PR quality check

• [ ] This is a new feature or a refactoring, and the PR is based against the main branch.
• [x] This is a bug fix, and the PR is based against the earliest maintained branch in which the bug can be reproduced.
• [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.