Unify type of credentials used in githubscm methods

Currently githubscm methods work with either credentials_id or token_credentials_id.

The real difference between the value is that credentials_id is username:password pair, whereas token_credentials_id is just string token. Moreover since GitHub stopped accepting passwords for authentication, in fact the username:password pair is currently username:token - thus the value hold by token_credentials_id is typically contained and accessible in matching credentials_id value.

This task should entail research of all applicable usages in githubscm (and pipelines using it) and evaluation if all can be replaced by credentials_id username:token pair.

It would simplify constructs as:
```
def checkoutIfExists(String repository, String author, String branches, String defaultAuthor, String defaultBranches, boolean mergeTarget = false, def credentials = ['token': 'kie-ci1-token', 'usernamePassword': 'kie-ci']) {
assert credentials['token']
assert credentials['usernamePassword']
def sourceAuthor = author
def sourceRepository = getForkedProjectName(defaultAuthor, repository, sourceAuthor, credentials['token']) ?: repository
// Checks source group and branch (for cases where the branch has been created in the author's forked project)
def repositoryScm = getRepositoryScm(sourceRepository, author, branches, credentials['usernamePassword'])
....

```
Notice the selective passing of correct credentials type to individual method based on how they're implemented.
It should be possible to rewrite the methods expecting token, and instead of:
```
withCredentials([string(credentialsId: credentialsId, variable: 'GITHUB_TOKEN')]) {
```
use
```
withCredentials([usernamePassword(credentialsId: credentialsId, usernameVariable: 'GITHUB_USER', passwordVariable: 'GITHUB_TOKEN')]) {
```
And just not using GITHUB_USER env var if not needed. But would simplify the implementation.