Update to Elixir 1.11+#2

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sampscl wants to merge 4 commits intomeyercm:masterfrom

Commits on Jun 2, 2021

* Removed :pg2, replaced with :pg equivalents; note :pg is the first child in Flub.App children
sampscl
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Commits on Jun 3, 2021

Commits on Sep 13, 2021

fixed startup when :pg is already running
sampscl
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