releng: bin/dist_push only pushes xenial packages (#3649)
releng: bin/dist_push only pushes xenial packages (#3649)

## Description
https://tyktech.atlassian.net/wiki/spaces/EN/pages/1180237826/Version+management+in+releng

This change needs to be merged to all release branches.

## Related Issue
https://tyktech.atlassian.net/browse/TD-553

## How This Has Been Tested
Tested in last handover to QA.

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] Refactoring or add test (improvements in base code or adds test coverage to functionality)

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes that apply -->
<!-- If you're unsure about any of these, don't hesitate to ask; we're here to help! -->
- [x] Make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). If pulling from your own
      fork, don't request your `master`!
- [x] Make sure you are making a pull request against the **`master` branch** (left side). Also, you should start
      *your branch* off *our latest `master`*.
- [ ] My change requires a change to the documentation.
  - [ ] If you've changed APIs, describe what needs to be updated in the documentation.
  - [ ] If new config option added, ensure that it can be set via ENV variable
- [ ] I have updated the documentation accordingly.
- [ ] Modules and vendor dependencies have been updated; run `go mod tidy && go mod vendor`
- [ ] When updating library version must provide reason/explanation for this update.
- [ ] I have added tests to cover my changes.
- [ ] All new and existing tests passed.
- [ ] Check your code additions will not fail linting checks:
  - [ ] `go fmt -s`
  - [ ] `go vet`

lonelycode/tyk,lonelycode/tyk,lonelycode/tyk