ARROW-15095: [Dev][Website] Changelog generation should use commit me…

…ssages

Display commit titles in the changelog instead of jira titles. Since we list all of the resolved jira tickets even without an applied patch, this is only possible where we have an issue-commit pair available.

I'm currently regenerating the whole changelog using the following command:

```bash
$ archery release --jira-cache /tmp/changelog changelog regenerate
Querying changelog for version: 6.0.1
Querying changelog for version: 6.0.0
Querying changelog for version: 5.0.0
Querying changelog for version: 4.0.1
Querying changelog for version: 4.0.0
...
```

Closes #12155 from kszucs/ARROW-15095

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>