Adjusted handling of the compressed tar archive file containing the kraken2 database in the process kraken2_db_preparation. So far it was assumed that the name of the contained folder within the tar archive equals the base name of the compressed tar archive file. For the provided MiniKraken2 database this is not always the case, i.e. sometimes only a prefix matches but without a consistent pattern (see also https://ccb.jhu.edu/software/kraken2/index.shtml?t=downloads).
Since I did not want to make any further assumptions on the folder name, I changed this to use all *.k2d in any contained folder, assuming each provided db file contains exactly one folder with *.k2d files.

Additionally I updated the links in the help and documentation to the minikraken2 database, which was recently updated.

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 necessary, also make a PR on the nf-core/mag branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: https://github.com/nf-core/mag/tree/master/.github/CONTRIBUTING.md