Remove PostgreSQL 11.9 from .tool-versions
What does this Merge Request do and why?
This MR removes postgres 11.9 from .tool-versions as discussed at !2255 (comment 731320868).
Merge Request checklist
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This change is backward compatible. If not, please include steps to communicate to our users. 
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Tests added for new functionality. If not, please raise Issue to follow-up. 
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Documentation added/updated, if needed. 
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gdk doctortest added, if needed.
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Add the ~highlightlabel if this MR should be included in theCHANGELOG.md.
Edited  by Ash McKenzie