chore: replace io/ioutil package

[ronething]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

As of Go 1.16, io/ioutil package is deprecated, and the same functionality is now provided by package io or package os, so we maybe can replace io/ioutil package.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #1485 (39ba77a) into master (ed92690) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1485   +/-   ##
=======================================
  Coverage   41.26%   41.26%           
=======================================
  Files          83       83           
  Lines        7365     7365           
=======================================
  Hits         3039     3039           
  Misses       3972     3972           
  Partials      354      354           

Impacted Files	Coverage Δ
pkg/apisix/cluster.go	36.12% <100.00%> (ø)
pkg/config/config.go	64.13% <100.00%> (ø)

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[ronething]

@tao12345666333 hi, can you please take a look, thx.

[tao12345666333]

Thanks!