[Backport 5.0] Fix Bitbucket Server exclude repo to use project key instead of name (#49837)

The excludable repo name should be created from the `Project.Key` (i.e.
`pjlast`) and not the `Project.Name` (i.e. `Petri-Johan Last`).

This fixes the `Exclude repository` button in the repo settings page not
necessarily working for Bitbucket Server repositories.

## Test plan

Tests updated

<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
-->
 <br> Backport af8da59 from #49829

Co-authored-by: Petri-Johan Last <[email protected]>

Author: github-actions[bot]

cmd/frontend/backend/external_services.go

@@ -221,7 +221,7 @@ func ExcludableRepoName(repository *types.Repo, logger log.Logger) (name string)
 			if repo.Project == nil {
 				return
 			}
-			name = fmt.Sprintf("%s/%s", repo.Project.Name, repo.Name)
+			name = fmt.Sprintf("%s/%s", repo.Project.Key, repo.Name)
 		} else {
 			logger.Error("invalid repo metadata schema", log.String("extSvcType", extsvc.TypeBitbucketServer))
 		}

cmd/frontend/backend/external_services_test.go

@@ -153,7 +153,7 @@ func makeBitbucketServerRepo() *types.Repo {
 		Slug: "jsonrpc2",
 		Project: &bitbucketserver.Project{
 			Key:  "SOURCEGRAPH",
-			Name: "SOURCEGRAPH",
+			Name: "Sourcegraph e2e testing",
 		},
 	}