Add Github Action badge and remove duplicated CI.

[wyk9787]

Fixes #540

10.3 is the oldest supported Xcode version. Support for any previous versions of Xcode is dropped. macOS 10.15 is the only supported macOS version now.

• Added Github Action badge.

• Removed AppVeyor and its badge.

• Removed most of the Travis CI script with the exception of Golang master version.

References of macOS and Xcode support:

• Github Action currently only supports macOS 10.15.

• Github Action's Xcode support: https://github.com/actions/virtual-environments/blob/master/images/macos/macos-10.15-Readme.md#xcode

• Travis CI's macOS & Xcode support: https://docs.travis-ci.com/user/reference/osx/#macos-version

[wyk9787]

The previous PR's Github Action passed: https://github.com/google/pprof/actions/runs/148062605.

Github Action's script takes effect as expected in this PR. It also includes the version (i.e. a few more XCode versions) that I added in this PR.

@aalexand Could you remove AppVeyor as required check for PR submission in the setting?

[kalyanac]

Could you remove AppVeyor as required check for PR submission in the setting?

Disabled AppVeyor webhook.

[codecov-commenter]

Codecov Report

Merging #547 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
+ Coverage   68.38%   68.41%   +0.03%     
==========================================
  Files          78       78              
  Lines       16088    16088              
==========================================
+ Hits        11001    11006       +5     
+ Misses       4228     4226       -2     
+ Partials      859      856       -3     

Impacted Files	Coverage Δ
internal/binutils/binutils.go	60.86% <0.00%> (+1.97%)	⬆️

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[wyk9787]

Could you remove AppVeyor as required check for PR submission in the setting?

Disabled AppVeyor webhook.

Thanks! Checks are now passing.

[kalyanac]

Can you mention in the description what is the oldest XCode and OSX versions this will support and what versions are getting dropped?

[wyk9787]

Can you mention in the description what is the oldest XCode and OSX versions this will support and what versions are getting dropped?

Done.

[aalexand]

The badge links to the image itself which is not useful and is uncommon to do. Please make the badge link to the CI status page, like the AppVeyor badge did and like Travis CI and Codecov badges do.

[wyk9787]

Sent #549.