fix(misc): rename last-delta schema to v2

[sherali42]

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rename last-delta-histogram to readable name delta-histogram-v2

[Copilot]

Pull request overview

This PR renames the last-delta-histogram schema to delta-histogram-v2 to improve naming clarity and readability. The old name last-delta-histogram was less intuitive, while delta-histogram-v2 better indicates it's an evolved version of the delta histogram schema.

• Renamed Scala variable lastDeltaHistogram to deltaHistogramV2 in the Schemas object
• Updated schema configuration key from last-delta-histogram to delta-histogram-v2
• Updated all references to use the new naming

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
core/src/main/scala/filodb.core/metadata/Schemas.scala	Updated the schema variable name from lastDeltaHistogram to deltaHistogramV2
core/src/main/resources/filodb-defaults.conf	Renamed schema configuration from last-delta-histogram to delta-histogram-v2 including the downsample-schema reference
gateway/src/main/scala/filodb/gateway/conversion/InputRecord.scala	Updated the schema reference in writeLastDeltaHistRecord to use deltaHistogramV2

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