[CALCITE-4312] Improve content of prepareVote draft email
[CALCITE-4312] Improve content of prepareVote draft email

1. Remove broken links.
2. Remove docker build instructions (there is no docker-compose file for Calcite).
3. Use GitHub URL to preview release notes.
4. Add subject template.

looker-open-source/calcite,julianhyde/calcite,apache/calcite,apache/calcite,julianhyde/calcite,julianhyde/calcite,apache/calcite,looker-open-source/calcite,jcamachor/calcite,apache/calcite,apache/calcite,looker-open-source/calcite,apache/calcite,julianhyde/calcite,jcamachor/calcite,looker-open-source/calcite,jcamachor/calcite,jcamachor/calcite,looker-open-source/calcite,jcamachor/calcite,julianhyde/calcite,julianhyde/calcite,jcamachor/calcite,looker-open-source/calcite