feat: migrate to using/urfave/cli/v2

[xiaodiannao]

Describe the PR
go get -u github.com/swaggo/swag/cmd/swag results in some errors because the https://github.com/urfave/cli implementation changed.

Snippet of the error message:

../github.com/swaggo/swag/cmd/swag/main.go:25:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
	cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
../github.com/swaggo/swag/cmd/swag/main.go:30:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
	cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)
../github.com/swaggo/swag/cmd/swag/main.go:35:16: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
	cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)

Additional context
Sorry for the number of commits, I edited directly on GitHub.

[easonlin404]

could you help change:

• imported github.com/urfave/cli to github.com/urfave/cli/v2
• executed go mod tidy and commited go.mod file

That will be work for this change. 😄

[codecov-io]

Codecov Report

Merging #618 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #618   +/-   ##
=======================================
  Coverage   85.84%   85.84%           
=======================================
  Files           7        7           
  Lines        1696     1696           
=======================================
  Hits         1456     1456           
  Misses        150      150           
  Partials       90       90

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[xiaodiannao]

Hi @easonlin404 thank you for your reply I updated it, now the checks are passing

[easonlin404]

LGTM