Cleanup Authentication Transports

[andygrunwald]

go-jira supports multiple ways for authentication (in the form of transports).

Goal of this task

Clean those up. Cleaning means

• Personal Access Token and Bearer are the same -> Choose the better name
• Only provide the ways of authentication that is supported by Cloud and OnPremise

[andygrunwald]

Might help:

• https://developer.atlassian.com/cloud/jira/platform/basic-auth-for-rest-apis/
• https://developer.atlassian.com/cloud/jira/platform/rest/v3/intro/#authentication