Release payjoin-1.0.0-rc.8

This release tightens the sender's validation of the sighash types declared on
its own inputs.

Selected Improvements:

Bug Fixes

• Require sender inputs to declare a sighash type that commits to all inputs
  and outputs. Only ECDSA SIGHASH_ALL, taproot
  SIGHASHDEFAULT/SIGHASH_ALL, and an unset type are accepted, both when
  building the sender context and when validating the receiver's proposal
  (#1790, #1793)

What's Changed

• Enable external-sources for shellcheck formatting. by @benalleng in #1785
• Keep receiver input failures recoverable by @Jolah1 in #1769
• Bump dart payjoin 0.2.0 for pub.dev by @spacebear21 in #1786
• Make the C# package README user-facing by @chavic in #1764
• Reject non-SIGHASH_ALL sender inputs in proposal by @spacebear21 in #1790
• update runners to Ubuntu 26.04 by @caarloshenriq in #1791
• Sighash all followups by @spacebear21 in #1793
• Prefer MSRV-compatible deps in the ubrn build by @spacebear21 in #1797
• Bump payjoin to version 1.0.0-rc.8 by @spacebear21 in #1795

Full Changelog: payjoin-1.0.0-rc.7...payjoin-1.0.0-rc.8