Update 21.4

[jaimergp]

Checklist

• [X] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [X] Reset the build number to 0 (if the version changed)
• [X] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [X] Ensured the license file is being packaged.

https://ambermd.org/bugfixes/AmberTools/21.0/update.4

 *******> update.4

 Author: Stephan Schott-Verdugo

 Date: July 16, 2021

 Programs: packmol-memgen

 Description: Fix v/v concentration of solutes added

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe:

• License is not an SPDX identifier (or a custom LicenseRef) nor an SPDX license expression.

Documentation on acceptable licenses can be found here.