Remove uses of pin_project::project attribute

[taiki-e]

pin-project will deprecate the project attribute due to some unfixable
limitations.

Refs: taiki-e/pin-project#225

Although the deprecation of the project attribute has not been released yet, this PR is submitted in advance to avoid the CI from being broken by deprecated warnings.

[codecov-commenter]

Codecov Report

Merging #1553 into master will decrease coverage by 0.10%.
The diff coverage is 36.11%.

@@            Coverage Diff             @@
##           master    #1553      +/-   ##
==========================================
- Coverage   50.95%   50.84%   -0.11%     
==========================================
  Files         126      133       +7     
  Lines       12041    12067      +26     
==========================================
  Hits         6136     6136              
- Misses       5905     5931      +26     

Impacted Files	Coverage Δ
actix-http/src/client/connection.rs	11.26% <0.00%> (+0.74%)	⬆️
actix-http/src/h2/dispatcher.rs	0.00% <0.00%> (ø)
src/responder.rs	83.76% <0.00%> (+0.43%)	⬆️
actix-http/src/service.rs	45.20% <33.33%> (+0.30%)	⬆️
actix-http/src/encoding/encoder.rs	81.81% <50.00%> (ø)
actix-http/src/body.rs	33.76% <62.50%> (+0.64%)	⬆️
actix-http/src/h1/dispatcher.rs	55.80% <71.42%> (+0.14%)	⬆️
actix-http/src/header/common/mod.rs	7.69% <0.00%> (-25.65%)	⬇️
actix-http/src/header/common/accept.rs	0.00% <0.00%> (ø)
actix-http/src/header/common/last_modified.rs	0.00% <0.00%> (ø)
... and 6 more

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[JohnTitor]

Thanks for the heads up!