[Go] Upgrade dependencies

[urandom2]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
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Description of the PR

antihax/optional#1 added module support, and was tagged as a stable
release. This change consumes that tag, and updates the psuedo-version
for golang.org/x/oauth2.

CC

@antihax @bvwells @grokify @kemokemo @bkabrda

Updates #4083.

[wing328]

@arnottcr thanks for the PR. I wonder if you can make the same change to the corresponding templates in the go-experimental generator as well:

• https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/go-experimental/go.mod.mustache
• https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/go-experimental/go.sum

[wing328]

On branch arnottcr-go-modules
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   samples/openapi3/client/petstore/go/go-petstore/go.mod
	modified:   samples/openapi3/client/petstore/go/go-petstore/go.sum

I'll update the outdated samples in another commit.

[wing328]

@arnottcr thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569