CLI-1107: Update confluent login environment variables

[brianstrauch]

Checklist

1. [CRUCIAL] Is the change for CP or CCloud functionalities that are already live in prod?

   • yes: ok

2. Did you add/update any commands that accept secrets as args/flags?

   • no: ok

What

Update the following environment variables:

• CCLOUD_EMAIL → CONFLUENT_CLOUD_EMAIL
• CCLOUD_PASSWORD → CONFLUENT_CLOUD_PASSWORD
• CONFLUENT_USERNAME → CONFLUENT_PLATFORM_USERNAME
• CONFLUENT_PASSWORD → CONFLUENT_PLATFORM_PASSWORD
• CONFLUENT_MDS_URL → CONFLUENT_PLATFORM_MDS_URL
• CONFLUENT_CA_CERT_PATH → CONFLUENT_PLATFORM_CA_CERT_PATH

References

CLI-1107

Test & Review

Test cases still pass. Manually verified that warnings are printed:

$ CCLOUD_EMAIL=bstrauch@confluent.io CCLOUD_PASSWORD=[REDACTED] confluent login
`CCLOUD_EMAIL` has been deprecated and replaced by `CONFLUENT_CLOUD_EMAIL`.
`CCLOUD_PASSWORD` has been deprecated and replaced by `CONFLUENT_CLOUD_PASSWORD`.

$ CONFLUENT_USERNAME=bstrauch@confluent.io CONFLUENT_PASSWORD=[REDACTED] CONFLUENT_MDS_URL=https://example.com:8090 CONFLUENT_CA_CERT_PATH=example.txt confluent login
`CONFLUENT_MDS_URL` has been deprecated and replaced by `CONFLUENT_PLATFORM_MDS_URL`.
`CONFLUENT_USERNAME` has been deprecated and replaced by `CONFLUENT_PLATFORM_USERNAME`.
`CONFLUENT_PASSWORD` has been deprecated and replaced by `CONFLUENT_PLATFORM_PASSWORD`.
`CONFLUENT_CA_CERT_PATH` has been deprecated and replaced by `CONFLUENT_PLATFORM_CA_CERT_PATH`.

[MuweiHe]

lgtm once ci passes