adding samtools version 1.21

[erinyoung]

There's a new version of samtools!

According to the release notes there are some bug fixes and feature updates.

I copied the file from 1.20.c and updated the version arg.

$ diff samtools/1.20.c/Dockerfile samtools/1.21/Dockerfile 
1c1
< ARG SAMTOOLS_VER="1.20"
---
> ARG SAMTOOLS_VER="1.21"

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• Include a description of what is in this pull request in this message.
• The dockerfile successfully builds to a test target for the user creating the PR. (i.e. docker build --tag samtools:1.15test --target test docker-builds/samtools/1.15 )
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• Program_Licenses.md contains the tool(s) used in this PR and has been updated for any missing

[erinyoung]

The samtools version in the README looks correct this time

[Kincekara]

Thank you for the update!
The deploy link: https://github.com/StaPH-B/docker-builds/actions/runs/11109718579