fix(credentials): proposal preview attribute

[TimoGlastra]

In the v2 preview credential_proposal should be credential_preview

[morrieinmaas]

LGTM!

[codecov-commenter]

Codecov Report

Merging #855 (6199e2b) into main (ba03fa0) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #855   +/-   ##
=======================================
  Coverage   87.59%   87.59%           
=======================================
  Files         463      463           
  Lines       11070    11070           
  Branches     1824     1824           
=======================================
  Hits         9697     9697           
  Misses       1311     1311           
  Partials       62       62           

Impacted Files	Coverage Δ
...entials/protocol/v2/CredentialFormatCoordinator.ts	89.06% <100.00%> (ø)
...les/credentials/protocol/v2/V2CredentialService.ts	89.25% <100.00%> (ø)
...protocol/v2/messages/V2ProposeCredentialMessage.ts	92.85% <100.00%> (ø)

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