Consul protocol version does not upgrade as documented

#### Overview of the Issue

According to the documentation the agents should upgrade to >2 protocol version automatically:

>`-protocol` - The Consul protocol version to use. Consul agents speak protocol 2 by default, however agents will automatically use protocol > 2 when speaking to compatible agents. __This should be set only when upgrading__. You can view the protocol versions supported by Consul by running consul `-v`.

https://www.consul.io/docs/agent/options#_protocol

But I've upgraded my whole cluster to `1.9.3` today and forced use of version 3 using `protocol` parameter in my JSON config, expecting it to stick. But when I attempt to remove the `protocol` config value set to 3 from a single host it reverts to using verison 2 after restart:
```
admin@node-01.consul.example.org:~ % consul members
Node Address Status Type Build Protocol DC Segment
node-01.consul.example.org 11.22.33.44:8301 alive server 1.9.3 2 ab-cde3
```
But the docs clearly state that __"This should be set only when upgrading."__, so why is it reverting?

#### Reproduction Steps

1. Switch all nodes - both servers and agents - to protocol version 3
1. Try to remove the `protocol: 3` the config on one of the hosts
1. See how it switched back to using version 3 with `consul members`

### Consul info for both Client and Server

Client info

```
agent:
check_monitors = 2
check_ttls = 0
checks = 6
services = 7
build:
prerelease =
revision = f55da930
version = 1.9.3
consul:
acl = disabled
known_servers = 3
server = false
runtime:
arch = amd64
cpu_count = 1
goroutines = 53
max_procs = 1
os = linux
version = go1.15.6
serf_lan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 240
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 5611
members = 39
query_queue = 0
query_time = 10
```

Server info

```
agent:
check_monitors = 1
check_ttls = 0
checks = 4
services = 4
build:
prerelease =
revision = f55da930
version = 1.9.3
consul:
acl = disabled
bootstrap = false
known_datacenters = 4
leader = false
leader_addr = 35.184.154.56:8300
server = true
raft:
applied_index = 25028587
commit_index = 25028587
fsm_pending = 0
last_contact = 49.00668ms
last_log_index = 25028587
last_log_term = 1308
last_snapshot_index = 25016263
last_snapshot_term = 1302
latest_configuration = [{Suffrage:Voter ID:2ec484a1-76c3-e368-cc83-2557bd6e32f7 Address:35.225.159.96:8300} {Suffrage:Voter ID:93086e4f-f4ca-aa40-bd58-602db5eca99e Address:104.197.126.101:8300} {Suffrage:Voter ID:a920fe0f-af66-4984-e67b-bb32630d9f8f Address:35.184.154.56:8300}]
latest_configuration_index = 0
num_peers = 2
protocol_version = 3
protocol_version_max = 3
protocol_version_min = 0
snapshot_version_max = 1
snapshot_version_min = 0
state = Follower
term = 1308
runtime:
arch = amd64
cpu_count = 1
goroutines = 225
max_procs = 1
os = linux
version = go1.15.6
serf_lan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 240
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 5611
members = 39
query_queue = 0
query_time = 10
serf_wan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 1
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 1516
members = 12
query_queue = 0
query_time = 1
```

### Operating system and Environment details

Ubuntu 20.04.1, amd64