NEXT-36018 - refactor: Unify SendMailAction constants

[aragon999]

1. Why is this change necessary?

Currently there are a duplicated constants, and when developing I was unsure which ones to use.

2. What does this change do, exactly?

Unify the constants and deprecate accordingly.

3. Describe each step to reproduce the issue or behaviour.

Develop something with mail attachments or mail actions.

4. Please link to the relevant issues (if any).

-

5. Checklist

• I have rebased my changes to remove merge conflicts
• I have written tests and verified that they fail without my change
• I have created a changelog file with all necessary information about my changes
• I have written or adjusted the documentation according to my changes
• This change has comments for package types, values, functions, and non-obvious lines of code
• I have read the contribution requirements and fulfil them.

[jozsefdamokos]

@aragon999 thanks for your contribution!