docker+docs: update documentation around Golang minimum version #16735

Job	Run time
Sqlc check	2m 40s
RPC and mobile compilation check	1m 49s
check commits	8m 38s
sample configuration check	29s
cross compilation	30m 38s
lint code	3m 11s
run ubuntu itests (btcd, backend=btcd cover=1)	32m 52s
run unit tests (btcd unit-cover)	20m 1s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	44m 12s
check pinned dependencies (google.golang.org/grpc v1.59.0)	5s
run unit tests (unit tags="kvdb_etcd")	21m 20s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	40m 53s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	4s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	43m 11s
run unit tests (unit tags="kvdb_postgres")	20m 10s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	54m 53s
run unit tests (unit tags="kvdb_sqlite")	18m 25s
run macOS itest	39m 5s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 8m 0s
run unit tests (btcd unit-race)	40m 44s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	42m 56s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	1h 11m 28s
run unit tests (unit-module)	3m 41s
check release notes updated	5s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	42m 17s
run ubuntu itests (neutrino, backend=neutrino cover=1)	39m 21s
run windows itest	7m 9s
finish	3s
	11h 38m 20s

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