Flip flags, upgrade Go (#289)

- Set --jobs=40 (was 100). This is a small repo running against a small
  cluster, so having too many jobs results in queueing.
- Set --grpc_keepalive_timeout=30s, as recommended by our UI.
- Set --nolegacy_important_outputs, same.
- Upgraded from Bazel 6.4.0 to 7.0.1. This enables --remote_download_minimal
  implicitly.
- Upgrade Go to 1.21.6.

.bazelrc

@@ -1,5 +1,5 @@
 # Options common for all EngFlow remote configurations.
-build:engflow_common --jobs=100
+build:engflow_common --jobs=40
 build:engflow_common --define=EXECUTOR=remote
 build:engflow_common --disk_cache=
 build:engflow_common --experimental_inmemory_dotd_files
@@ -9,7 +9,9 @@ build:engflow_common --remote_timeout=3600
 build:engflow_common --action_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1
 build:engflow_common --java_runtime_version=remotejdk_11
 build:engflow_common --java_language_version=11
-build:engflow_common --remote_download_toplevel
+build:engflow_common --grpc_keepalive_time=30s
+build:engflow_common --legacy_important_outputs=false
+build:engflow_common --remote_download_minimal
 
 # Options for continuous testing.
 build:ci --color=yes

WORKSPACE

@@ -212,7 +212,7 @@ go_dependencies()
 
 go_rules_dependencies()
 
-go_register_toolchains(version = "1.21.5")
+go_register_toolchains(version = "1.21.6")
 
 gazelle_dependencies()