isHidden was a `Boolean` in order to treat a special case identified
with V1 templates where if the create index request didn't specify if
the index should be hidden or not (ie. isHidden was `null`) but the
index matched a template that specified the `index.hidden` setting we
needed to remove the global templates from the templates we'll apply to
the new index (note: this is important with V1 templates as inheritance
is supported).

With V2 templates we match only one template with an index so the
equivalent check did not need to exist (we added a sanity check in
elastic#55015 where we make sure
we don't apply an invalid global template - one that specifes the
`index.hidden` setting, but this is a check we make irrespective of the
user specifying or not if the index should be hidden)

This commit makes `isHidden` when matching V2 templates a boolean
primitive, eliminating the need for the `null` state to exist. Note that
some methods which use the matching V2 templates still work with a
`Boolean` object `isHidden` attribute as they are also matching the V1
templates. These methods will pass in `false` instead of `null` when
finding the V2 templates.

(cherry picked from commit c5b923a)
Signed-off-by: Andrei Dan <andrei.dan@elastic.co>