Rename payjoin-service to payjoin-mailroom

[spacebear21]

Rebrand!

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Pull Request Test Coverage Report for Build 22021916730

Details

• 5 of 8 (62.5%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.18%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-mailroom/src/main.rs	0	3	0.0%

Totals
Change from base Build 22020902563:	0.0%
Covered Lines:	10217
Relevant Lines:	12283

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💛 - Coveralls