fix ioutil deprecation, make oauth github host configurable

[hendrikKahl]

How to categorize this PR?

/area testing
/kind technical-debt

What this PR does / why we need it:

Update all calls to deprecated io/ioutil package and replace them with corresponding functions from os / io package. See https://pkg.go.dev/io/ioutil@go1.17.13 for details.

Add a new, optional field to the github oauth configuration for the testmachinery bot. This will enable support for instances other than github.com. If not specified, it will default to github.com.

A bot config for another github instance would look like this:

apiVersion: config.testmachinery.gardener.cloud/v1beta1
kind: BotConfiguration
dashboard:
  authentication:
    cookieSecret: abc
    githubConfig:
      oAuth:
        clientId: abc
        clientSecret: abc
        redirectUrl: https://tm-bot.test/oauth/redirect
      organization: my-org
      hostname: some-github-instance.com
    provider: github

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
/invite @dguendisch

Release note:

testmachinery bot supports configuration of github hostname for oauth.

[dguendisch]

/lgtm