php7.3-memcached should now be available

[rfay]

Describe the bug

memcached for php 7.3 has been released, so we should get it into our next release.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Version and configuration information (please complete the following information):

• Host computer OS and Version: [e.g. Windows 10, macOS High Sierra]
• Docker version information (use docker version) and insert it here.
• ddev version information (use ddev version)
• config.yaml contents for the misbehaving project
• Do you have any custom configuration (nginx, php, mysql) in the .ddev folder? If so, have you tried without them?

Additional context
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