[Backport 9.0] Revert "Uppercase enum source mode"

[github-actions]

Backport 779fb2a from #3810.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
indices.add_block	🟢	2/2	2/2
indices.analyze	🟢	243/243	243/243
indices.cancel_migrate_reindex	🟢	2/2	2/2
indices.clear_cache	🟢	4/4	4/4
indices.clone	🟢	5/5	5/5
indices.close	🟢	49/49	49/49
indices.create_data_stream	🟢	31/31	31/31
indices.create_from	🟢	4/4	4/4
indices.create	🔴	1092/1176	1176/1176
indices.data_streams_stats	🟢	4/4	4/4
indices.delete_alias	🔴	12/12	5/12
indices.delete_data_lifecycle	⚪	Missing test	Missing test
indices.delete_data_stream	🟢	28/28	28/28
indices.delete_index_template	🟢	1/1	1/1
indices.delete_template	🟢	9/9	9/9
indices.delete	🟢	105/105	105/105
indices.disk_usage	🟢	6/6	6/6
indices.downsample	🟢	2/2	2/2
indices.exists_alias	🟢	37/37	37/37
indices.exists_index_template	⚪	Missing test	Missing test
indices.exists_template	🟢	15/15	15/15
indices.exists	🟢	39/39	39/39
indices.explain_data_lifecycle	⚪	Missing test	Missing test
indices.field_usage_stats	🟢	5/5	5/5
indices.flush	🟢	41/41	41/41
indices.forcemerge	🟢	12/12	12/12
indices.get_alias	🟢	68/68	68/68
indices.get_data_lifecycle_stats	⚪	Missing test	Missing test
indices.get_data_lifecycle	⚪	Missing test	Missing test
indices.get_data_stream	🟢	11/11	11/11
indices.get_field_mapping	🟢	15/15	15/15
indices.get_index_template	🔴	21/21	17/21
indices.get_mapping	🔴	135/135	133/135
indices.get_migrate_reindex_status	🟢	1/1	1/1
indices.get_settings	🔴	76/76	66/76
indices.get_template	🟢	32/32	32/32
indices.get	🟢	55/55	55/55
indices.migrate_reindex	🟢	2/2	2/2
indices.migrate_to_data_stream	⚪	Missing test	Missing test
indices.modify_data_stream	⚪	Missing test	Missing test
indices.open	🟢	20/20	20/20
indices.promote_data_stream	⚪	Missing test	Missing test
indices.put_alias	🔴	58/58	20/58
indices.put_data_lifecycle	⚪	Missing test	Missing test
indices.put_index_template	🔴	59/68	68/68
indices.put_mapping	🔴	111/117	117/117
indices.put_settings	🔴	55/58	58/58
indices.put_template	🔴	45/46	46/46
indices.recovery	🟢	9/9	9/9
indices.refresh	🟢	275/275	275/275
indices.reload_search_analyzers	🟢	2/2	2/2
indices.resolve_cluster	🟢	13/13	13/13
indices.resolve_index	🟢	6/6	6/6
indices.rollover	🔴	26/26	1/26
indices.segments	🔴	6/6	5/6
indices.shard_stores	🔴	5/5	4/5
indices.shrink	🟢	5/5	5/5
indices.simulate_index_template	🔴	6/7	5/7
indices.simulate_template	🔴	5/5	4/5
indices.split	🟢	11/11	11/11
indices.stats	🔴	113/113	30/112
indices.update_aliases	🔴	26/26	7/26
indices.validate_query	🟢	12/12	12/12

You can validate these APIs yourself by using the make validate target.