refactor: move from io/ioutil to io and os package

[Juneezee]

The io/ioutil package has been deprecated in Go 1.16 (See https://golang.org/doc/go1.16#ioutil). This PR replaces the existing io/ioutil functions with their new definitions in io and os packages.

This PR is partly related to #16764.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@aa631d8). Click here to learn what that means.
The diff coverage is 41.23%.

@@           Coverage Diff           @@
##             main   #17109   +/-   ##
=======================================
  Coverage        ?   45.31%           
=======================================
  Files           ?      773           
  Lines           ?    86873           
  Branches        ?        0           
=======================================
  Hits            ?    39364           
  Misses          ?    41147           
  Partials        ?     6362           

Impacted Files	Coverage Δ
cmd/dump.go	0.91% <0.00%> (ø)
models/repo.go	54.44% <0.00%> (ø)
modules/context/repo.go	52.99% <0.00%> (ø)
modules/doctor/paths.go	14.49% <0.00%> (ø)
modules/git/repo_commit.go	42.35% <0.00%> (ø)
modules/git/repo_tree_nogogit.go	49.01% <0.00%> (ø)
modules/graceful/server.go	52.30% <0.00%> (ø)
modules/indexer/code/elastic_search.go	1.65% <0.00%> (ø)
modules/markup/external/external.go	1.61% <0.00%> (ø)
modules/migrations/restore.go	0.00% <0.00%> (ø)
... and 46 more

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