Consistent approach for sensitive value masking in CLIs

### Body

We agreed via [LAZY CONSENSUS](https://lists.apache.org/thread/3dhzqvpw4z2x0wfokpmdncsjwws86zcs) that we will not espose sensitive information over the public API (exception is task-sdk API).

This is a meta-issue describing what needs to be done. Sub-issues are created to complete the work.

This means:

1) we want to make it crystal clear that no APIs ever expose sensitive data

2) we should remove export (import can stay) via UI - and leave a
comment that export is only available via local CLI

3) the "sensitive data not exposed over API" is also present in
airflow-ctl - this means that airflow-ctl should never expose
sensitive data (including connections, variables, config, export)

4) the "expose config" [5] - will only accept "false" and
"non-sensitive-only". The "true" will be rejected.

There is also an impact to local CLI, even if local CLI user has
access to all data anyway:

5) local CLI * list (connections, variables, config) only by default
returns "keys" - and it will only return values when `--show-values`
is passed as command line option (with clear comment in help that this
option **might** show sensitive data, also when we do `* list` command
without `--show-values` we emit stderr output explaining that
potentially sensitive data is hidden and you need to specify
`--show-values` to see them

6) the local CLI * get commands are unaffected (those are more likely
already used as CLI API

7) we remove connections list --conn-id as it is equivalent to connections get

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.

Start by reviewing the sub-issues for the UI, airflow-ctl, local CLI list commands, and config exposure. Check the existing CLI help and behavior for connections, variables, config, and export. Done means sensitive values are hidden by default, explicit show-values behavior is documented, unsafe config exposure is rejected, and the connections list --conn-id option is removed.

Written by the indexing model from the issue text.