Fix incorrect dtype for electrodes table filtering (float -> text)

[rly]

Summary of changes

• The electrodes table column "filtering" has dtype "float32" but is meant to store freeform text. The doc says "Description of hardware filtering, including the filter name and frequency cutoffs." The dtype should be "text".

PR checklist for schema changes

• Update the version string in docs/format/source/conf.py, core/nwb.namespace.yaml, and /core/nwb.file.yaml
  to the next version with the suffix "-alpha"
• Add a new section in the release notes for the new version with the date "Upcoming"
• Add release notes for the PR to docs/format/source/format_release_notes.rst