[Backport 9.2] 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	🟢	558/558 → 578/578	576/576 → 596/596
esql.query	🟢 → 🔴	359/359 → 361/363	0/0
indices.create	🔴	1412/1440 → 1432/1460	1440/1440 → 1460/1460
indices.create_data_stream	🟢	131/131 → 132/132	131/131 → 132/132
indices.downsample	🟢 → 🔴	9/9 → 40/42	9/9 → 42/42
indices.get	🟢	66/66 → 69/69	66/66 → 69/69
indices.get_data_stream	🔴	124/124 → 125/125	77/124 → 77/125
indices.get_mapping	🔴	213/213 → 225/225	202/213 → 214/225
indices.get_settings	🔴	86/86 → 93/93	66/86 → 73/93
indices.put_index_template	🔴	140/164 → 142/166	164/164 → 166/166
indices.put_settings	🔴	56/58 → 78/80	58/58 → 80/80
indices.segments	🟢 → 🔴	5/5 → 6/6	5/5 → 5/6
ingest.put_pipeline	🟢	78/78 → 79/79	78/78 → 79/79
search	🔴	2581/2615 → 2628/2659	2615/2615 → 2659/2659

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