Add dependabot to `.mailmap`.

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>
```