fix: add upper bound of HTTPX version (#347)

- HTTPX 0.27 supports both the `proxies` and `proxy` args, while version
0.28 supports only the `proxy`.
- I am adopting the updated arg name to align with the newer version. 
- However, we cannot fully upgrade to 0.28 as there are more issues.

Author: vdusek

docs/02-guides/01-requests-and-httpx.mdx

@@ -92,7 +92,7 @@ async def main():
     async with Actor:
         proxy_configuration = await Actor.create_proxy_configuration()
         proxy_url = await proxy_configuration.new_url()
-        async with httpx.AsyncClient(proxies=proxy_url) as httpx_client:
+        async with httpx.AsyncClient(proxy=proxy_url) as httpx_client:
             response = httpx_client.get(f'http://example.com'),
             print(response)
 ```

docs/03-concepts/05-proxy-management.mdx

@@ -185,9 +185,9 @@ use the [`proxies` argument](https://www.python-httpx.org/advanced/#http-proxyin
 proxy_configuration = await Actor.create_proxy_configuration()
 proxy_url = await proxy_configuration.new_url()
 
-response = httpx.get('http://example.com', proxies=proxy_url)
+response = httpx.get('http://example.com', proxy=proxy_url)
 # --- OR ---
-async with httpx.AsyncClient(proxies=proxy_url) as httpx_client:
+async with httpx.AsyncClient(proxy=proxy_url) as httpx_client:
     response = await httpx_client.get('http://example.com')
 ```