Expected behavior:
tsh login should correctly assume default proxy port or at least print a helpful error message.

Current behavior:
tsh login fails to correctly assume port 443 and prints a confusing message:

tsh login --proxy=cluster.example.com --auth=github
ERROR: Teleport proxy not available at proxy address cluster.example.com:3080 or cluster.example.com:443
	context deadline exceeded

Bug details:
Teleport version: 15.0.0-alpha.5
Recreation steps:

1. Create a cluster in the FIPS mode, configure Github connector (not sure about which part exactly causes the issue). Proxy service config:

proxy_service:
  enabled: "yes"
  web_listen_addr: 0.0.0.0:443
  public_addr: cluster.example.com:443
  https_keypairs: []
  https_keypairs_reload_interval: 0s
  acme:
    enabled: "yes"
    email: someuser@example.com

2. Attempt to sign in with tsh login --proxy=cluster.example.com --auth=github: failure
3. Attempt to sign in with tsh login --proxy=cluster.example.com:443 --auth=github: success