BREAKING CHANGE: xDnsServerMx: Remove resource

[johlju]

Pull Request (PR) description

• xDnsServerMx
  • BREAKING CHANGE: The resource has been replaced by DnsServerMx (issue BREAKING CHANGE: xDnsRecordMx: Remove deprecated resource #228).

Preparation for renaming module.

This Pull Request (PR) fixes the following issues

• Fixes BREAKING CHANGE: xDnsRecordMx: Remove deprecated resource #228

Task list

• Added an entry to the change log under the Unreleased section of the
  file CHANGELOG.md. Entry should say what was changed and how that
  affects users (if applicable), and reference the issue being resolved
  (if applicable).
• Resource documentation added/updated in README.md.
• Resource parameter descriptions added/updated in README.md, schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[johlju]

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[johlju]

Reviewed 9 of 9 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johlju)