Update of CM pipeline settings doesn't validate branch/tag existence

**Expected Behavior**

User input (branch/tag/repositoryId) must be validated upon update of CM pipeline settings.

**Actual Behavior**

`aio cloudmanager:pipeline:update` ends successfully for not existing branches/tags.

**Reproduction Scenario, Platform, and Version**

```
$ aio -v
@adobe/aio-cli/8.1.0 darwin-x64 node-v14.17.6
```

**Steps to Reproduce**

```
$ aio cloudmanager:pipeline:update PIPELINEID -p PROGRAMID --branch=this-branch-does-not-exist
updating pipeline... updated pipeline ID

$ aio cloudmanager:pipeline:update PIPELINEID -p PROGRAMID --tag=not-existing-tag
updating pipeline... updated pipeline ID
```

Once above is set you can start the pipeline, but it's gonna fail immediately at validation stage with the following message:

> Failed: Invalid pipeline: . Reason=Branch=this-branch-does-not-exist not found in repository (Finished: September 22, 2021 11:14:23 AM GMT+2)

In contrast repository ID is being validated, so I assume that it may require some backend API changes:

```
$ aio cloudmanager:pipeline:update PIPELINEID -p PROGRAMID --repositoryId=123456
updating pipeline... failed
› Error: [CloudManagerSDK:ERROR_UPDATE_PIPELINE] Cannot update pipeline: https://cloudmanager.adobe.io/api/program/PROGRAMID/pipeline/PIPELINEID (400 Bad Request) - Validation
› Error(s): Referenced repository not found in application or is not ready for use: 123456
› Code: ERROR_UPDATE_PIPELINE
```