Add dependabot to .mailmap.

Before:
```
$ git shortlog -se | grep "depend"
    44	dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
     6	dependabot-preview[bot] <dependabot-preview[bot]@users.noreply.github.com>
     4	dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
    74	dependabot[bot] <dependabot[bot]@users.noreply.github.com>
    93	dependabot[bot] <support@dependabot.com>
```
After:
```
$ git shortlog -se | grep "depend"
   221  dependabot[bot] <dependabot[bot]@users.noreply.github.com>
```

Author: nnethercote

.mailmap

@@ -141,6 +141,10 @@ David Ross <daboross@daboross.net>
 David Wood <david@davidtw.co> <david.wood@huawei.com>
 Deadbeef <ent3rm4n@gmail.com>
 Deadbeef <ent3rm4n@gmail.com> <fee1-dead-beef@protonmail.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <27856297+dependabot-preview[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <49699333+dependabot[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <dependabot-preview[bot]@users.noreply.github.com>
+dependabot[bot] <dependabot[bot]@users.noreply.github.com> <support@dependabot.com>
 Derek Chiang <derekchiang93@gmail.com> Derek Chiang (Enchi Jiang) <derekchiang93@gmail.com>
 DeveloperC <DeveloperC@protonmail.com>
 Devin Ragotzy <devin.ragotzy@gmail.com>