Update release script JSON schema find/replace to be compatible with macOS (#19962)

BSD vs GNU `sed` problems:
```shell
$ sed -i '0,/^\$id: .*/s||$id: https://element-hq.github.io/synapse/schema/synapse/v1.157/synapse-config.schema.json|' schema/synapse-config.schema.yaml
sed: 1: "schema/synapse-config.s ...": bad flag in substitute command: 'h'
```

changelog.d/19962.misc

@@ -0,0 +1 @@
+Update release script JSON schema find/replace task to be compatible with macOS.

scripts-dev/release.py

@@ -31,6 +31,7 @@ import sys
 import time
 import urllib.request
 from os import path
+from pathlib import Path
 from tempfile import TemporaryDirectory
 from typing import Any
 
@@ -257,10 +258,16 @@ def _prepare() -> None:
     subprocess.check_output(["poetry", "version", new_version])
 
     # Update config schema $id.
-    schema_file = "schema/synapse-config.schema.yaml"
+    schema_file_path = Path("schema/synapse-config.schema.yaml")
     major_minor_version = ".".join(new_version.split(".")[:2])
     url = f"https://element-hq.github.io/synapse/schema/synapse/v{major_minor_version}/synapse-config.schema.json"
-    subprocess.check_output(["sed", "-i", f"0,/^\\$id: .*/s||$id: {url}|", schema_file])
+    # Find/replace the `$id: ...` line in `schema/synapse-config.schema.yaml` with a new
+    # unique identifier for this release
+    schema_file_content = schema_file_path.read_text()
+    new_schema_file_content = re.sub(
+        r"^\$id: .*", f"$id: {url}", schema_file_content, count=1, flags=re.MULTILINE
+    )
+    schema_file_path.write_text(new_schema_file_content)
 
     # Generate changelogs.
     generate_and_write_changelog(synapse_repo, current_version, new_version)