Bump payjoin to version 1.0.0-rc.8

[spacebear21]

Prepare the payjoin-1.0.0-rc.8 release per #1794. Bumps the crate version
and every workspace member's requirement on it, refreshes both lock files,
and adds the changelog section for the changes merged since rc.7.

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.

Disclosure: co-authored by Claude Opus 5

[coveralls]

Coverage Report for CI Build 31031432110

Coverage decreased (-0.04%) to 86.685%

Details

• Coverage decreased (-0.04%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 6 coverage regressions across 2 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

6 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
payjoin-cli/src/app/v2/mod.rs	3	58.22%
payjoin-cli/src/app/v2/ohttp.rs	3	52.0%

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Coverage Stats

Relevant Lines:	16418
Covered Lines:	14232
Line Coverage:	86.69%
Coverage Strength:	344.53 hits per line

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💛 - Coveralls

[benalleng]

ACK b313b97