chore: Avoid one clone and use better iterators

[Hywan]

The first patch uses slice::from_ref to create a &[T] from a &T. In our case, it saves the cost of one clone.

The second patch uses values() instead of iter() and discard the keys.

---

• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.92%. Comparing base (f4b2794) to head (ab60a3a).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/room/knock_requests.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6557      +/-   ##
==========================================
- Coverage   89.93%   89.92%   -0.02%     
==========================================
  Files         381      381              
  Lines      106865   106865              
  Branches   106865   106865              
==========================================
- Hits        96112    96095      -17     
- Misses       7101     7117      +16     
- Partials     3652     3653       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

---

_{Comparing Hywan:fix-clippy (ab60a3a) with main (6a33b94)}

[stefanceriu]

Using a somewhat unknown module and method to avoid a string clone is pushing it but sure, why not 😁
Could've easily been part of any other PR I reckon.