admin/render_readmes: Use async reqwest client and diesel-async queries

[Turbo87]

This should be our last place that still used a sync reqwest client :)

I tested this locally by hardcoding cdn_prefix: Some("https://static.crates.io".to_string()) and then running cargo run --bin crates-admin render-readmes --crate mustache --page-size 5. seemed to work fine from what I could tell.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 81 lines in your changes missing coverage. Please review.

Project coverage is 89.00%. Comparing base (23fa915) to head (669b247).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/crates-admin/render_readmes.rs	0.00%	81 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9892   +/-   ##
=======================================
  Coverage   88.99%   89.00%           
=======================================
  Files         290      290           
  Lines       30096    30095    -1     
=======================================
  Hits        26785    26785           
+ Misses       3311     3310    -1     

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[eth3lbert]

LGTM 👍 !

We could further utilize async-compression and tokio-tar and remove spawn_blocking() in a future PR.