[DOCS] Updates CHANGELOG for 8.10.0 release

CHANGELOG.md

@@ -1,5 +1,42 @@
 *To see release notes for the `7.x` branch and older releases, see [CHANGELOG on the 7.17 branch](https://github.com/elastic/elasticsearch-ruby/blob/7.17/CHANGELOG.md).*
 
+## 8.10.0 Release notes
+
+### Client
+* Tested versions of Ruby for 8.10.0: Ruby (MRI) 3.0, 3.1 and 3.2. JRuby 9.3 and JRuby 9.4.
+
+### API
+
+#### New Experimental APIs, for internal use:
+- `fleet.delete_secret`
+- `fleet.get_secret`
+- `fleet.post_secret`
+
+#### New stable APIs:
+- `security.get_settings` - Retrieve settings for the security system indices
+- `security.update_settings` - Update settings for the security system indices
+
+#### New Experimental API:
+- `query_ruleset.list` List query rulesets.
+
+#### API Changes:
+- `indices.reload_search_analyzers` - Adds parameter `resource` changed resource to reload analyzers from if applicable
+
+Promoted from Experimental to Beta:
+- `security.create_cross_cluster_api_key`
+- `security.update_cross_cluster_api_key`
+
+#### Synonyms namespace update:
+
+All synonym related APIs have been moved to the `synonyms` namespace and some of the endpoints have been renamed, as well as their parameters:
+- `synonyms.delete` => `synonyms.delete_synonym` - requires `id`, the id of the synonyms set to be deleted.
+- `synonyms.get` => `synonyms.get_synonym` - requires `id`, the name of the synonyms set to be retrieved.
+- `synonyms_set.get_synonyms_sets` => `synonyms.get_synonyms_sets`
+- `synonyms.put` => `synonyms.put_synonym` - requires `id` of the synonyms set to be created or updated.
+- `synonym_rule.put` => `synonyms.put_synonym_rule` - Parameters changed to `set_id` (the id of the synonym set to be updated with the synonym rule) and `rule_id` (the id of the synonym rule to be updated or created).
+- New Experimental API `synonyms.delete_synonym_rule` - Deletes a synonym rule in a synonym set
+- New Experimental API `synonyms.get_synonym_rule` - Retrieves a synonym rule from a synonym set
+
 ## 8.9.0 Release notes
 
 ### Client