fix(oauth): use ruma::time::instant for wasm compatibility

[Johennes]

This switches from std::time::Instant to ruma::time::Instant because the former doesn't work under Wasm.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.62%. Comparing base (2f7d2b3) to head (639eae8).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5889   +/-   ##
=======================================
  Coverage   88.61%   88.62%           
=======================================
  Files         362      362           
  Lines      102814   102814           
  Branches   102814   102814           
=======================================
+ Hits        91113    91116    +3     
+ Misses       7468     7464    -4     
- Partials     4233     4234    +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5889 will not alter performance

_{Comparing Johennes:johannes/ruma-instant (639eae8) with main (2f7d2b3)}

Summary

✅ 50 untouched

[poljar]

Yup, makes sense.