Regex update to avoid over-redaction of GitHub issues

[bismuthsalamander]

Hello - this PR adjusts the regexes found in airflow/include/tasks/extract/github.py that remove boilerplate text from GitHub issue thread text. Before this change, the regular expressions will remove too much text due to the greedy matching.

Consider the following example:

Discussion here.
<!--\r\nThank you. http://chris.beams.io/posts/git-commit/\r\n-->
More discussion here.
<!--\r\nThank you. http://chris.beams.io/posts/git-commit/\r\n-->
Even more discussion here

The two lines containing comments should be removed, but the greedy match in the regular expression <!--\r\nThank you.*?http://chris.beams.io/posts/git-commit/\r\n--> will cause the line More discussion here. to be removed as well.

To fix that behavior, this PR replaces each greedy .* sequence with a lazy .*? sequence so that the minimum (intended) match is removed.