update docs

UPDATING.md

@@ -24,6 +24,7 @@ assists people when migrating to a new version.
 
 ## Next
 
+- [31844](https://github.com/apache/superset/pull/31844) The `ALERT_REPORTS_EXECUTE_AS` and `THUMBNAILS_EXECUTE_AS` config parameters have been renamed to `ALERT_REPORTS_EXECUTORS` and `THUMBNAILS_EXECUTORS` respectively. A new config flag `CACHE_WARMUP_EXECUTORS` has also been introduced to be able to control which user is used to execute cache warmup tasks. Finally addition, the config flag `THUMBNAILS_SELENIUM_USER` has been removed. To use a fixed executor for async tasks, use the new `FixedExecutor` class. See the config and docs for more info on setting up different executor profiles.
 - [31894](https://github.com/apache/superset/pull/31894) Domain sharding is deprecated in favor of HTTP2. The `SUPERSET_WEBSERVER_DOMAINS` configuration will be removed in the next major version (6.0)
 - [31774](https://github.com/apache/superset/pull/31774): Fixes the spelling of the `USE-ANALAGOUS-COLORS` feature flag. Please update any scripts/configuration item to use the new/corrected `USE-ANALOGOUS-COLORS` flag spelling.
 - [31582](https://github.com/apache/superset/pull/31582) Removed the legacy Area, Bar, Event Flow, Heatmap, Histogram, Line, Sankey, and Sankey Loop charts. They were all automatically migrated to their ECharts counterparts with the exception of the Event Flow and Sankey Loop charts which were removed as they were not actively maintained and not widely used. If you were using the Event Flow or Sankey Loop charts, you will need to find an alternative solution.

docs/docs/configuration/alerts-reports.mdx

@@ -177,10 +177,9 @@ By default, Alerts and Reports are executed as the owner of the alert/report obj
 just change the config as follows (`admin` in this example):
 
 ```python
-from superset.tasks.types import ExecutorType
+from superset.tasks.types import FixedExecutor
 
-THUMBNAIL_SELENIUM_USER = 'admin'
-ALERT_REPORTS_EXECUTE_AS = [ExecutorType.SELENIUM]
+ALERT_REPORTS_EXECUTORS = [FixedExecutor("admin")]
 ```
 
 Please refer to `ExecutorType` in the codebase for other executor types.

docs/docs/configuration/cache.mdx

@@ -94,10 +94,9 @@ By default thumbnails are rendered per user, and will fall back to the Selenium
 To always render thumbnails as a fixed user (`admin` in this example), use the following configuration:
 
 ```python
-from superset.tasks.types import ExecutorType
+from superset.tasks.types import FixedExecutor
 
-THUMBNAIL_SELENIUM_USER = "admin"
-THUMBNAIL_EXECUTE_AS = [ExecutorType.SELENIUM]
+THUMBNAIL_EXECUTORS = [FixedExecutor("admin")]
 ```
 
 
@@ -130,8 +129,6 @@ def init_thumbnail_cache(app: Flask) -> S3Cache:
 
 
 THUMBNAIL_CACHE_CONFIG = init_thumbnail_cache
-# Async selenium thumbnail task will use the following user
-THUMBNAIL_SELENIUM_USER = "Admin"
 ```
 
 Using the above example cache keys for dashboards will be `superset_thumb__dashboard__{ID}`. You can