dnsdist: Require Quiche >= 0.15.0

[rgacogne]

Short description

Based on the release notes the API we use did not change after that.
Fixes #13433

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 6704954598

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 436 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.2%) to 57.44%

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Files with Coverage Reduction	New Missed Lines	%
pdns/sstuff.hh	1	61.99%
pdns/backends/gsql/gsqlbackend.hh	2	95.43%
pdns/dnsdist.cc	2	70.8%
pdns/misc.hh	3	82.52%
pdns/recursordist/syncres.hh	3	63.05%
pdns/stubresolver.cc	3	81.63%
pdns/validate.cc	4	60.61%
pdns/misc.cc	10	62.51%
pdns/mtasker.cc	11	71.2%
pdns/recursordist/test-syncres_cc1.cc	12	88.91%

Totals
Change from base Build 6704520672:	-0.2%
Covered Lines:	105422
Relevant Lines:	152943

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💛 - Coveralls