chore!: always use async walredo, warn if sync is configured (#7754)

refs https://github.com/neondatabase/neon/issues/7753

This PR is step (1) of removing sync walredo from Pageserver.

Changes:
* Remove the sync impl
* If sync is configured, warn! and use async instead
* Remove the metric that exposes `kind`
* Remove the tenant status API that exposes `kind`

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

After we've released this change to prod and are sure we won't
roll back, we will

1. update the prod Ansible to remove the config flag from the prod
   pageserver.toml.
2. remove the remaining `kind` code in pageserver

These two changes need no release inbetween.

See  https://github.com/neondatabase/neon/issues/7753 for details.

test_runner/regress/test_pageserver_config.py

@@ -1,35 +0,0 @@
-import pytest
-from fixtures.neon_fixtures import (
-    NeonEnvBuilder,
-    last_flush_lsn_upload,
-)
-
-
-@pytest.mark.parametrize("kind", ["sync", "async"])
-def test_walredo_process_kind_config(neon_env_builder: NeonEnvBuilder, kind: str):
-    neon_env_builder.pageserver_config_override = f"walredo_process_kind = '{kind}'"
-    # ensure it starts
-    env = neon_env_builder.init_start()
-    # ensure the metric is set
-    ps_http = env.pageserver.http_client()
-    metrics = ps_http.get_metrics()
-    samples = metrics.query_all("pageserver_wal_redo_process_kind")
-    assert [(s.labels, s.value) for s in samples] == [({"kind": kind}, 1)]
-    # ensure default tenant's config kind matches
-    # => write some data to force-spawn walredo
-    ep = env.endpoints.create_start("main")
-    with ep.connect() as conn:
-        with conn.cursor() as cur:
-            cur.execute("create table foo(bar text)")
-            cur.execute("insert into foo select from generate_series(1, 100)")
-    last_flush_lsn_upload(env, ep, env.initial_tenant, env.initial_timeline)
-    ep.stop()
-    ep.start()
-    with ep.connect() as conn:
-        with conn.cursor() as cur:
-            cur.execute("select count(*) from foo")
-            [(count,)] = cur.fetchall()
-            assert count == 100
-
-    status = ps_http.tenant_status(env.initial_tenant)
-    assert status["walredo"]["process"]["kind"] == kind