SNOW-1571918 Update deprecated usages to recommended ones (#1400)

Fixes warnings by updating uses of deprecated code:
- Fixes a warning from `click` that `autocompletion` is deprecated:
> DeprecationWarning: 'autocompletion' is renamed to 'shell_complete'. The old name is deprecated and will be removed in Click 8.1. See the docs about 'Parameter' for information about new behavior.
    _typer_param_setup_autocompletion_compat(self, autocompletion=autocompletion)

- Fixes a warning from `pydantic` about a deprecated pattern:
> PydanticDeprecatedSince20: Using extra keyword arguments on `Field` is deprecated and will be removed. Use `json_schema_extra` instead. (Extra keys: 'is_discriminator_field'). Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.8/migration/

src/snowflake/cli/api/commands/flags.py

@@ -163,7 +163,7 @@ def callback(value):
     ),
     show_default=False,
     rich_help_panel=_CONNECTION_SECTION,
-    autocompletion=lambda: list(get_all_connections()),
+    shell_complete=lambda _, __, ___: list(get_all_connections()),
 )
 
 TemporaryConnectionOption = typer.Option(

src/snowflake/cli/api/project/schemas/updatable_model.py

@@ -187,7 +187,8 @@ def DiscriminatorField(*args, **kwargs):  # noqa N802
     When this `DiscriminatorField` is used on a pydantic attribute,
     we will not allow templating on it.
     """
-    return Field(is_discriminator_field=True, *args, **kwargs)
+    extra = dict(is_discriminator_field=True)
+    return Field(json_schema_extra=extra, *args, **kwargs)
 
 
 def IdentifierField(*args, **kwargs):  # noqa N802