Regex update to avoid over-redaction of GitHub issues (#325)

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.

airflow/include/tasks/extract/github.py

@@ -214,12 +214,12 @@ def extract_github_issues(repo_base: str, github_conn_id: str, cutoff_date: str
     issues_drop_text = [
         dedent(
             """                     <\\!--\r
-                         .*Licensed to the Apache Software Foundation \\(ASF\\) under one.*under the License\\.\r
+                         .*?Licensed to the Apache Software Foundation \\(ASF\\) under one.*?under the License\\.\r
                           -->"""
         ),
         "<!-- Please keep an empty line above the dashes. -->",
-        "<!--\r\nThank you.*http://chris.beams.io/posts/git-commit/\r\n-->",
-        r"\*\*\^ Add meaningful description above.*newsfragments\)\.",
+        "<!--\r\nThank you.*?http://chris.beams.io/posts/git-commit/\r\n-->",
+        r"\*\*\^ Add meaningful description above.*?newsfragments\)\.",
     ]
 
     issue_markdown_template = dedent(