pageserver: only write out legacy tenant config if no generation (#6891)

## Problem

Previously we always wrote out both legacy and modern tenant config
files. The legacy write enabled rollbacks, but we are long past the
point where that is needed.

We still need the legacy format for situations where someone is running
tenants without generations (that will be yanked as well eventually),
but we can avoid writing it out at all if we do have a generation number
set. We implicitly also avoid writing the legacy config if our mode is
Secondary (secondary mode is newer than generations).

## Summary of changes

- Make writing legacy tenant config conditional on there being no
generation number set.

test_runner/fixtures/neon_fixtures.py

@@ -3812,7 +3812,7 @@ def pytest_addoption(parser: Parser):
 
 
 SMALL_DB_FILE_NAME_REGEX: re.Pattern = re.compile(  # type: ignore[type-arg]
-    r"config|config-v1|heatmap-v1|metadata|.+\.(?:toml|pid|json|sql|conf)"
+    r"config-v1|heatmap-v1|metadata|.+\.(?:toml|pid|json|sql|conf)"
 )
 
 

test_runner/regress/test_tenant_conf.py

@@ -299,8 +299,7 @@ def test_creating_tenant_conf_after_attach(neon_env_builder: NeonEnvBuilder):
 
     # tenant is created with defaults, as in without config file
     (tenant_id, timeline_id) = env.neon_cli.create_tenant()
-    config_path = env.pageserver.tenant_dir(tenant_id) / "config"
-    assert config_path.exists(), "config file is always initially created"
+    config_path = env.pageserver.tenant_dir(tenant_id) / "config-v1"
 
     http_client = env.pageserver.http_client()