[Backport 8.19] Allow single percent in PercentilesAggregation

[github-actions]

Backport 8bef67e from #5552.

[github-actions]

Following you can find the validation changes against the target branch for the APIs.

API	Status	Request	Response
bulk	🟢	517/517 → 537/537	535/535 → 555/555
indices.create	🔴	1217/1280 → 1237/1300	1280/1280 → 1300/1300
indices.create_data_stream	🟢	116/116 → 117/117	116/116 → 117/117
indices.downsample	🟢	6/6 → 36/36	6/6 → 36/36
indices.get	🟢	66/66 → 68/68	66/66 → 68/68
indices.get_alias	🔴	89/89 → 93/93	80/89 → 81/93
indices.get_data_stream	🔴	115/115 → 116/116	17/115 → 17/116
indices.get_mapping	🔴	178/178 → 189/189	165/178 → 176/189
indices.get_settings	🔴	84/84 → 90/90	65/84 → 71/90
indices.put_index_template	🔴	110/142 → 112/144	142/142 → 144/144
indices.put_settings	🔴	56/58 → 78/80	58/58 → 80/80
indices.segments	🟢 → 🔴	5/5 → 7/7	5/5 → 5/7
indices.unfreeze	⚪ → 🟢	Missing test → 1/1	Missing test → 1/1
ingest.put_pipeline	🟢	73/73 → 74/74	73/73 → 74/74
search	🔴	2285/2326 → 2327/2365	2326/2326 → 2365/2365

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