Make `mapping.single_type` only settable if the index is a 5.x index

[s1monw]

mapping.single_type allows to add multiple types which is something that 6.x indices must not allow. Yet, today this setting is still modifiable while it defaults to true In order to move make it only settable on 5.x indices and read-only in 6.0 we need to fix several things like:

• Fix PercolatorFieldMapperTests to use a single type PercolatorFieldMapperTests needs to be rewritten to only use a single type. #24958 / Stop using the mapping.single_type setting in percolator tests. #24963
• Move Rest tests that use multiple types into a mixed cluster scenario such that indices that are tested against are created with a 5.x version. Skip rest tests that use mutiple types in pure 6.x clusters #24965
• Remove this setting from AbstractQueryTestCase
• disallow setting index.mapper.single_type on 6.x indices Ensure index.mapping.single_type can only be set on 5.x indices #25375
• Fix Parent/Child to work without multiple types Typeless parent/child #20257
• Fix Documentation do not use multiple types Fix documentation to not use multiple types #25401

[nik9000]

Move Rest tests that use multiple types into a mixed cluster scenario such that indices that are tested against are created with a 5.x version.

The qa:full-cluster-restart project might be useful here.

[s1monw]

The qa:full-cluster-restart project might be useful here.

I don't think it needs to be that complicated we can just use the qa:mixed-cluster project