docs: Update AWS documentation

[alexchen8]

• Remove AWS_CREDENTIAL_EXPIRATION reference which seems to be a leftover from Rusoto migration.
• Remove warning about credentials process now that it's implemented in the SDK. Confirmed in 0.24.1

  2022-10-01T07:43:17.176046Z DEBUG send_operation{operation="HeadBucket" service="s3"}:provide_credentials{provider=default_chain}:load_credentials{provider=Profile}:load_credentials_file: aws_config::profile::parser::source: config file loaded path=~/.aws/credentials size=175
  2022-10-01T07:43:17.176214Z  INFO send_operation{operation="HeadBucket" service="s3"}:provide_credentials{provider=default_chain}:load_credentials{provider=Profile}: aws_config::profile::credentials: constructed abstract provider from config file chain=ProfileChain { base: CredentialProcess("xxxxx ** arguments redacted **"), chain: [] }
  2022-10-01T07:43:17.176240Z  INFO send_operation{operation="HeadBucket" service="s3"}:provide_credentials{provider=default_chain}:load_credentials{provider=Profile}: aws_config::profile::credentials::exec: first credentials will be loaded from CredentialProcess("xxxxx ** arguments redacted **") base=CredentialProcess("xxxxx ** arguments redacted **")
  2022-10-01T07:43:17.176332Z DEBUG send_operation{operation="HeadBucket" service="s3"}:provide_credentials{provider=default_chain}:load_credentials{provider=Profile}:load_base_credentials: aws_config::credential_process: loading credentials from external process command=xxxxx ** arguments redacted **
  2022-10-01T07:43:17.564104Z  INFO send_operation{operation="HeadBucket" service="s3"}:provide_credentials{provider=default_chain}:load_credentials{provider=Profile}: aws_config::profile::credentials: loaded base credentials creds=Credentials { provider_name: "CredentialProcess", access_key_id: "xxxxxxxx", secret_access_key: "** redacted **", expires_after: "2022-10-01T15:07:43Z" }
  

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[spencergilbert]

Thanks for catching that!