Fix priority not being set when updating MX records

[hmoffatt]

Description

Since #1170 , it is not possible to update the priority of an MX record. The priority is not sent in the PATCH update request. Also discussed in #1282 .

Has your change been tested?

I've tested this manually with flarectl.

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[github-actions]

changelog detected ✅

[codecov-commenter]

Codecov Report

Merging #1290 (f0f5570) into master (a5dc232) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1290   +/-   ##
=======================================
  Coverage   49.07%   49.07%           
=======================================
  Files         134      134           
  Lines       13091    13091           
=======================================
  Hits         6425     6425           
  Misses       5162     5162           
  Partials     1504     1504           

Impacted Files	Coverage Δ
dns.go	45.05% <ø> (ø)

[github-actions]

This functionality has been released in v0.68.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!