Switch scripts to use bash

[jasontedor]

This commit switches the command-line scripts to use bash instead of sh
so that we can take advantage of features that bash provides like
arrays.

Closes #14002

[nik9000]

I still like it. Quoting things in sh is pain. bash's arrays make this so much nicer.

[jasontedor]

Quoting things in sh is pain. bash's arrays make this so much nicer.

That's exactly what is motivating this PR.

[dakrone]

I know this is already merged, but can we add bash as a dependency for our RPM and DEB packages now?

[dakrone]

I also think this should be marked as "breaking" and have a note in the migration guide

[nik9000]

Both of those things are good. It is "breaking" for folks that are trying to support Elasticsearch outside of our packages. Some folks will have to react and that'll help.

[jasontedor]

I know this is already merged, but can we add bash as a dependency for our RPM and DEB packages now?

I also think this should be marked as "breaking" and have a note in the migration guide

I opened #18259.