resource/github_repository: Avoid spurious diff for topics

[radeksimko]

Topics don't really have an order, so they should be a set, not a list.

This will avoid spurious diff when they're not in alphabetical order (which is common case when you combine two or more variables).

TF_ACC=1 go test ./github -v -run=TestAccGithubRepository_ -timeout 120m
=== RUN   TestAccGithubRepository_basic
--- PASS: TestAccGithubRepository_basic (5.29s)
=== RUN   TestAccGithubRepository_archive
--- PASS: TestAccGithubRepository_archive (3.27s)
=== RUN   TestAccGithubRepository_archiveUpdate
--- PASS: TestAccGithubRepository_archiveUpdate (4.93s)
=== RUN   TestAccGithubRepository_importBasic
--- PASS: TestAccGithubRepository_importBasic (2.99s)
=== RUN   TestAccGithubRepository_defaultBranch
--- PASS: TestAccGithubRepository_defaultBranch (6.54s)
=== RUN   TestAccGithubRepository_templates
--- PASS: TestAccGithubRepository_templates (3.51s)
=== RUN   TestAccGithubRepository_topics
--- PASS: TestAccGithubRepository_topics (6.62s)
=== RUN   TestAccGithubRepository_autoInitForceNew
--- PASS: TestAccGithubRepository_autoInitForceNew (6.16s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	39.330s

State migration is not required, because Read can just keep setting []string and Set() will do its magic.