add elgato 1.21.0

[Kincekara]

Closes #1250
I updated the version argument, added a test for the pdf report, and updated the readme for it.

Diff:

diff build-files/elgato/1.20.2/Dockerfile build-files/elgato/1.21.0/Dockerfile
6c6
< ARG ELGATO_VER="1.20.2"
---
> ARG ELGATO_VER="1.21.0"
59a60,62
> 
> # test reporting
> RUN elgato_report.py -i test/report.json -o test/report.pdf
diff build-files/elgato/1.20.2/README.md build-files/elgato/1.21.0/README.md
32a33,35
> 
> # PDF report
> elgato_report.py -i report.json -o report.pdf 

Pull Request (PR) checklist:

• 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/build-files/samtools/1.15 )
• Directory structure as name of the tool in lower case with special characters removed with a subdirectory of the version number in build-files (i.e. docker-builds/build-files/spades/3.12.0/Dockerfile)
  • (optional) All test files are located in same directory as the Dockerfile (i.e. build-files/shigatyper/2.0.1/test.sh)
• Create a simple container-specific README.md in the same directory as the Dockerfile (i.e. docker-builds/build-files/spades/3.12.0/README.md)
  • If this README is longer than 30 lines, there is an explanation as to why more detail was needed
• Dockerfile includes the recommended LABELS
• Main README.md has been updated to include the tool and/or version of the dockerfile(s) in this PR
• Program_Licenses.md contains the tool(s) used in this PR and has been updated for any missing