remove neon_local --pageserver-config-override (#7614)

Preceding PR https://github.com/neondatabase/neon/pull/7613 reduced the
usage of `--pageserver-config-override`.

This PR builds on top of that work and fully removes the `neon_local
--pageserver-config-override`.

Tests that need a non-default `pageserver.toml` control it using two
options:

1. Specify `NeonEnvBuilder.pageserver_config_override` before
`NeonEnvBuilder.init_start()`. This uses a new `neon_local init
--pageserver-config` flag.
2. After `init_start()`: `env.pageserver.stop()` +
`NeonPageserver.edit_config_toml()` + `env.pageserver.start()`

A few test cases were using
`env.pageserver.start(overrides=("--pageserver-config-override...",))`.
I changed them to use one of the options above. 

Future Work
-----------

The `neon_local init --pageserver-config` flag still uses `pageserver
--config-override` under the hood. In the future, neon_local should just
write the `pageserver.toml` directly.

The `NeonEnvBuilder.pageserver_config_override` field should be renamed
to `pageserver_initial_config`. Let's save this churn for a separate
refactor commit.

test_runner/performance/test_branch_creation.py

@@ -140,10 +140,14 @@ def test_branch_creation_many(neon_compare: NeonCompare, n_branches: int, shape:
 
     # start without gc so we can time compaction with less noise; use shorter
     # period for compaction so it starts earlier
+    def patch_default_tenant_config(config):
+        tenant_config = config.get("tenant_config", {})
+        tenant_config["compaction_period"] = "3s"
+        tenant_config["gc_period"] = "0s"
+        config["tenant_config"] = tenant_config
+
+    env.pageserver.edit_config_toml(patch_default_tenant_config)
     env.pageserver.start(
-        overrides=(
-            "--pageserver-config-override=tenant_config={ compaction_period = '3s', gc_period = '0s' }",
-        ),
         # this does print more than we want, but the number should be comparable between runs
         extra_env_vars={
             "RUST_LOG": f"[compaction_loop{{tenant_id={env.initial_tenant}}}]=debug,info"