feat(migrations): add migration from Bitbucket Server (Data Center) (#39913)

Add migration from Bitbucket Server (Data Center). It has no issues,
wiki, milestones or releases, so those are not offered.

Migrated: git data with tags and LFS, and pull requests with state,
draft flag, merge commit/base, comments, approvals and inline code
comments including resolved threads. Multi-line ranges become a `Lines
a-b:` text prefix, as Gitea anchors code comments to one line. Auth is
an HTTP access token (personal, project or repository) or
username/password; 429 responses are retried.

Changes outside the downloader:
- `ReviewComment.Resolved`, applied by the uploader.
- The uploader creates a fork PR's head branch from its head commit when
already in the repository, instead of fetching the fork (Bitbucket keeps
it in `refs/pull-requests/<id>/from`; private forks need credentials).

Tested with unit tests, responses recorded from Bitbucket 10.5.1, and an
end-to-end migration. Independent of
https://github.com/go-gitea/gitea/pull/38607 (Bitbucket Cloud); the
service type enum value needs adjusting for whichever lands second.

---------

Co-authored-by: bircni <bircni@icloud.com>

routers/web/repo/migrate.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"net/http"
 	"net/url"
+	"slices"
 
 	admin_model "gitea.dev/models/admin"
 	"gitea.dev/models/db"
@@ -231,8 +232,13 @@ func setMigrationContextData(ctx *context.Context, serviceType structs.GitServic
 	ctx.Data["DisableNewPullMirrors"] = setting.Mirror.DisableNewPull
 
 	// Plain git should be first
-	ctx.Data["Services"] = append([]structs.GitServiceType{structs.PlainGitService}, structs.SupportedFullGitService...)
+	services := append([]structs.GitServiceType{structs.PlainGitService}, structs.SupportedFullGitService...)
+	ctx.Data["Services"] = slices.DeleteFunc(services, func(gt structs.GitServiceType) bool {
+		variants := gt.Variants()
+		return len(variants) > 0 && variants[0] != gt
+	})
 	ctx.Data["service"] = serviceType
+	ctx.Data["ServiceVariants"] = serviceType.Variants()
 }
 
 func MigrateRetryPost(ctx *context.Context) {