Consolidate and cleanup clippy configuration

[steviez]

Problem

#5673 added clippy.toml when we already had .clippy.toml. The new file was getting automatically ignored:

warning: using config file `/.../solana/.clippy.toml`, `/.../solana/clippy.toml` will be ignored

and contained invalid lint configuration:

error: error reading Clippy's configuration file: unknown field `deny-attributes`, expected one of
...

Summary of Changes

• Move invalid config in clippy.toml to Cargo.toml
• Move contents of .clippy.toml to clippy.toml

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.3%. Comparing base (c64126f) to head (8ef0328).
⚠️ Report is 3659 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5710   +/-   ##
=======================================
  Coverage    83.3%    83.3%           
=======================================
  Files         824      824           
  Lines      373748   373748           
=======================================
+ Hits       311538   311549   +11     
+ Misses      62210    62199   -11     

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