Migrate to golang 1.20

[jmhbnz]

Migrate main to golang 1.20, as 1.19 is now out of support.

One task towards #16393

Ran some benchmarks between this branch and main locally as follows:

# Server
GOMAXPROCS=10 ./bin/etcd --quota-backend-bytes=17179869184 --listen-client-urls http://0.0.0.0:23790 --advertise-client-urls http://127.0.0.1:23790

# Benchmark
GOMAXPROCS=6 go run ./tools/benchmark put --conns=64 --clients=64 --key-size=256 --val-size=4096 --key-space-size=50000 --total=2000000 --endpoints "http://127.0.0.1:23790"

# make clean && make build between each run

	Run 1	Run 2	Run 3
this branch etcd Version: 3.6.0-alpha.0 Go Version: go1.20.7 Go OS/Arch: linux/amd64	Summary: Total: 468.3909 secs. Slowest: 0.0711 secs. Fastest: 0.0026 secs. Average: 0.0150 secs. Stddev: 0.0065 secs. Requests/sec: 4269.9374	Summary: Total: 468.5622 secs. Slowest: 0.0873 secs. Fastest: 0.0030 secs. Average: 0.0150 secs. Stddev: 0.0065 secs. Requests/sec: 4268.3772	Summary: Total: 467.0949 secs. Slowest: 0.0751 secs. Fastest: 0.0025 secs. Average: 0.0149 secs. Stddev: 0.0065 secs. Requests/sec: 4281.7851
main etcd Version: 3.6.0-alpha.0 Go Version: go1.19.12 Go OS/Arch: linux/amd64	Summary: Total: 469.1479 secs. Slowest: 0.0775 secs. Fastest: 0.0030 secs. Average: 0.0150 secs. Stddev: 0.0066 secs. Requests/sec: 4263.0482	Summary: Total: 467.0527 secs. Slowest: 0.0659 secs. Fastest: 0.0025 secs. Average: 0.0149 secs. Stddev: 0.0065 secs. Requests/sec: 4282.1723	Summary: Total: 467.0527 secs. Slowest: 0.0659 secs. Fastest: 0.0025 secs. Average: 0.0149 secs. Stddev: 0.0065 secs. Requests/sec: 4282.1723

[fuweid]

LGTM

[ahrtr]

@jmhbnz please rebase this PR, thx