Today if an Elasticsearch node reaches a disk watermark then it will repeatedly
emit logging about it, which implies that some action needs to be taken by the
administrator. This is misleading. Elasticsearch strives to keep nodes under
the high watermark, but it is normal to have a few nodes occasionally exceed
this level. Nodes may be over the low watermark for an extended period without
any ill effects.

This commit enhances the logging emitted by the `DiskThresholdMonitor` to be
less misleading. The expected case of hitting the high watermark and
immediately relocating one or more shards that to bring the node back under the
watermark again is reduced in severity to `INFO`. Additionally, `INFO` messages
are not emitted repeatedly.

Fixes elastic#48038

…logging