Local Nextclade dataset and update strict syntax for nf >=26

[nhhaidee]

• Update strict syntax for Nextflow 26
• Add options for local Nextclade dataset (Allow usage of local Nextclade datasets #123). Nextclade dataset can be downloaded here (https://github.com/nhhaidee/nf-test-datasets/blob/master/nf-flu/nextclade_flu_dataset.zip) to bypass the network download
• Require Nextflow >= 25.04.0

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
  • If you've added a new tool - ensure that you've added the version info to a versions.yml and added it to the ch_versions channel in the workflow you added the tool to.
  • If you've added a new tool - have you followed the pipeline conventions in the contribution docs
  • If necessary, also make a PR on the CFIA-NCFAD/nf-test-datasets repo
  • If appropriate, ensure that the illumina, nanopore and assemblies workflows have been updated to use the new tool.
  • Add or update an nf-test for the module/workflow/etc.
• Ensure the test suite passes (nextflow run . -profile test_{illumina,nanopore},docker).
• Ensure that the nf-test tests pass.
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).