Require token-based authentication

[wilkinsona]

When there's a token configured, it's used as the value of the password:

github-changelog-generator/src/main/java/io/spring/githubchangeloggenerator/github/service/GitHubProperties.java

Line 51 in 3e01a00

this.password = StringUtils.hasText(token) ? token : password;

When deciding whether to configure authentication, there has to be a username:

github-changelog-generator/src/main/java/io/spring/githubchangeloggenerator/github/service/GitHubService.java

Lines 60 to 62 in 3e01a00

	if (StringUtils.hasLength(username)) {
	builder = builder.basicAuthentication(username, password);
	}

With token-based authentication, a username isn't necessary. Instead, we should present the token as part of an Authentication: Bearer header.

[wilkinsona]

GitHub no longer supports username and password based authentication. The password has to be a token, and when it is, the username is ignored. We should adapt to this by dropping support for github.username and github.password and requiring github.token instead.