Upgrade EMQX Docker images to run on Erlang/OTP 26.

EMQX had been running on Erlang/OTP 26 since 5.5 except for docker images which were on Erlang/OTP 25.
Now all releases are on Erlang/OTP 26.

A known issue:
When an older version EMQX joins cluster with newer version nodes.
The older version node's schema registry may encounter an issue which emits logs like below:

```
Error loading module '$schema_parser___CiYAWBja87PleCyKZ58h__SparkPlug_B_BUILT-IN':,
This BEAM file was compiled for a later version of the runtime system than the current (Erlang/OTP 25).
```

This issue is fixed in newer version, however for older versions, a manual step is required.
Execute this in one of the clustered nodes before the older version EMQX joins the cluster.

```shell
emqx eval 'lists:foreach(fun(Key) -> mnesia:dirty_delete(emqx_ee_schema_registry_protobuf_cache_tab, Key) end, mnesia:dirty_all_keys(emqx_ee_schema_registry_protobuf_cache_tab)).'
```

Or if the older version EMQX is already in the cluster, execute the above command, and restart this node.