feat: add support for compression level -2 (HuffmanOnly)

The docappender is rejecting `-2` as a value for `CompressionLevel` despite it being a valid value for gzip compression in the standard library.

The validation logic should be updated to accept `-2`: https://github.com/elastic/go-docappender/blob/c563b1848e65b82232bae1f60e6eaeb2a08980c8/appender.go#L86

Open appender.go around line 86 and inspect the CompressionLevel validation logic. Update the accepted values so -2 is treated as valid, then verify that the docappender no longer rejects HuffmanOnly compression.

Written by the indexing model from the issue text.