FIX: checkout correct branches of plugin and discourse (#182)

[fzngagan]

• FIX: checkout correct branches of plugin and discourse

• add condition to check correct discourse branch

• FIX: use updated redis version

• bump patch version

[fzngagan]

@angusmcleod Here's a PR which cherrypicks the workflow updates to stable.

• Discourse branch checked out stable
• Plugin branch checkout out stable

Also, we've uncovered another bug here. We initiated a .stable suffix for stable branch versions to be able to distinguish between the main and stable branches. But turns out x.y.z.stable isn't a valid semver version.