refactor: lowercase channel factory across subworkflows + workflows

[tpall]

Summary

Pure cosmetic refactor: switch from the Channel class qualifier to the lowercase channel factory function across all subworkflows + workflows. Both forms work in DSL2; lowercase channel is the documented default and the more common style in modern nf-core pipelines.

This was one of the changes bundled into the now-closed #472 per @madeline-scyphers' note ("thanks for some of the other QoL updates like updating some of the syntax to DSL2 (Channel -> channel, etc.)"). Splitting it out into its own PR per the maintainer-friendly path agreed when refiling #503.

What changed

21e5cb8f refactor: lowercase `channel` factory across subworkflows + workflows

+42 / -42 lines, 7 files, mechanical Channel.X → channel.X substitution wherever a channel factory is used:

subworkflows/local/call.nf
subworkflows/local/collect_rna.nf
subworkflows/local/db_search.nf
subworkflows/local/merge.nf
subworkflows/local/qc.nf
subworkflows/local/utils_nfcore_dram_pipeline/main.nf
workflows/dram.nf

Test plan

• No semantic change — channel and Channel resolve to the same Nextflow factory in DSL2.
• git diff --stat shows equal +/- counts (one-for-one substitution).
• CI / integration test on dev if you'd like a smoke run before merge.

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