Update to version v0.20.13 of ndt-server

[cristinaleonr]

This PR updates ndt-server to version v0.20.13, which removes the bigquery:"-" tag from the ConnectionInfo field in the Measurement struct exported by ndt-server (https://github.com/m-lab/ndt-server/pull/368).

Commands run:

$ go get -u github.com/m-lab/ndt-server
$ go mod tidy

Related to: https://github.com/m-lab/dev-tracker/issues/731.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7406

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 67.227%

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Files with Coverage Reduction	New Missed Lines	%
active/active.go	2	90.63%
<!--	Total:	2		-->

Totals
Change from base Build 7403:	-0.04%
Covered Lines:	3321
Relevant Lines:	4940

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💛 - Coveralls