Do not prefix already prefixed format with 8

[pgomulka]

It happens when a locale is specified on a mapping that a new
JavaDateFormatter is created with a new mapping and already prefixed
with 8 format is prefixed again. This should change avoids this
behavior.

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[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)

[pgomulka]

discovered with yml test failure #47983

[pgomulka]

@elasticmachine run elasticsearch-ci/packaging-sample-matrix

[rjernst]

Looks fine. Just to double check: we would have never serialized multiple 8's right?

[pgomulka]

@elasticmachine run elasticsearch-ci/2

[pgomulka]

@rjernst good point, when serialising, it probably won't (I can't see how this could happen)
But when deserialising it would deserialise with double 8
added a testcase that fails without the change in JavaDateFormatter