2025-12-31: Remove support for legacy sharding

[EdSchouten]

Back in February we added support for rendezvous hashing to our sharding backend. To ease migration, we kept support for the old algorithm. Now that enough time has passed and assuming most people have migrated to the new algorithm, let's remove the old one.

Cc @meroton-benjamin

[aspect-workflows]

Test

25 test targets passed

Targets

//pkg/auth:auth_test [k8-fastbuild]                                                           967ms
//pkg/blobstore/buffer:buffer_test [k8-fastbuild]                                             71ms
//pkg/blobstore/completenesschecking:completenesschecking_test [k8-fastbuild]                 75ms
//pkg/blobstore/grpcclients:grpcclients_test [k8-fastbuild]                                   61ms
//pkg/blobstore/grpcservers:grpcservers_test [k8-fastbuild]                                   76ms
//pkg/blobstore/local:local_test [k8-fastbuild]                                               106ms
//pkg/blobstore/mirrored:mirrored_test [k8-fastbuild]                                         62ms
//pkg/blobstore/readcaching:readcaching_test [k8-fastbuild]                                   59ms
//pkg/blobstore/readfallback:readfallback_test [k8-fastbuild]                                 62ms
//pkg/blobstore/replication:replication_test [k8-fastbuild]                                   101ms
//pkg/blobstore/sharding/integration:integration_test [k8-fastbuild]                          160ms
//pkg/blobstore/sharding:sharding_test [k8-fastbuild]                                         920ms
//pkg/blobstore:blobstore_test [k8-fastbuild]                                                 105ms
//pkg/builder:builder_test [k8-fastbuild]                                                     87ms
//pkg/capabilities:capabilities_test [k8-fastbuild]                                           56ms
//pkg/digest:digest_test [k8-fastbuild]                                                       58ms
//pkg/filesystem/path:path_test [k8-fastbuild]                                                70ms
//pkg/grpc:grpc_test [k8-fastbuild]                                                           67ms
//pkg/http/server:server_test [k8-fastbuild]                                                  60ms
//pkg/jmespath:jmespath_test [k8-fastbuild]                                                   362ms
//pkg/jwt:jwt_test [k8-fastbuild]                                                             87ms
//pkg/otel:otel_test [k8-fastbuild]                                                           57ms
//pkg/prometheus:prometheus_test [k8-fastbuild]                                               50ms
//pkg/util:util_test [k8-fastbuild]                                                           73ms
//pkg/x509:x509_test [k8-fastbuild]                                                           53ms

Total test execution time was 4s. 4 tests (13.8%) were fully cached saving 359ms.