Replace mailgun adapter with mock adapter

[mtrezza]

New Pull Request Checklist

• I am not disclosing a vulnerability.
• I am creating this PR in reference to an issue.

Issue Description

Parse Server has a dependency on parse-server-simple-mailgun-adapter, which poses several issues. Parse Server uses it just for adapter loading tests.

Related issue: closes #7320

Approach

Replaced dependency on parse-server-simple-mailgun-adapter with new MockMailAdapter sub-package inside Parse Server package.

This is a breaking change, because the adapter has been part of Parse Server and therefore a mail adapter that was available without adding it manually as dependency. For sake of clean-up for the next release, this will be a breaking change without phased deprecation.

TODOs before merging

• Add test cases
• Add entry to changelog
• Add changes to documentation (guides, repository pages, in-code descriptions)

[codecov]

Codecov Report

Merging #7321 (34e2e00) into master (7042552) will decrease coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 34e2e00 differs from pull request most recent head b11cc20. Consider uploading reports for the commit b11cc20 to get more accurate results

@@            Coverage Diff             @@
##           master    #7321      +/-   ##
==========================================
- Coverage   93.93%   93.89%   -0.04%     
==========================================
  Files         181      181              
  Lines       13194    13194              
==========================================
- Hits        12394    12389       -5     
- Misses        800      805       +5     

Impacted Files	Coverage Δ
src/Options/Definitions.js	100.00% <ø> (ø)
src/Options/index.js	100.00% <ø> (ø)
src/Adapters/AdapterLoader.js	95.83% <0.00%> (-4.17%)	⬇️
src/batch.js	91.37% <0.00%> (-1.73%)	⬇️
src/ParseServerRESTController.js	97.01% <0.00%> (-1.50%)	⬇️
...dapters/Storage/Postgres/PostgresStorageAdapter.js	95.36% <0.00%> (-0.16%)	⬇️

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[davimacedo]

LGTM!

[mtrezza]

Note: The adapter has to be removed from README before merge.

[mtrezza]

Removed simple-mailgun-adapter from README

Also cleaned up the passwordPolicy and accountLockout options in README and improved their docs in Parse Server Options, because they were entangled and poorly documented.

[parseplatformorg]

🎉 This change has been released in version 5.0.0-beta.1

[parseplatformorg]

🎉 This change has been released in version 5.0.0