Generated server code with Go v1.15 and above generates deprecation warning for TLSConfig

## Problem statement

When generating server code the generated server.go produces a warning at this line:
httpsServer.TLSConfig.BuildNameToCertificate()

The warning is that this function has been deprecated since Go 1.14

## Steps to reproduce
run swagger generate server -f swagger.yaml

## Environment
swagger version: dev 
go version: 1.16.3
OS:   Debian Linux 10 (buster)
