Replace `GENCODE_DOWNLOAD` for nf-core `WGET`

[atrigila]

The current GENCODE_DOWNLOAD process can be replaced by the nf-core WGET module, which provides equivalent download functionality.

Some benefits include:

• The WGET module is tested and used across pipelines
• Supports adding custom parameters via ext.args
• Reduces maintenance and simplifies the code

Requirements

• Remove the GENCODE_DOWNLOAD process
• Integrate the nf-core WGET module -> we might need to have one for the GTF and one for the FASTA
• Update workflow connections and documentation
• Validate that downloads work correctLy