Prepare version v0.22.0 (#714)

Collect the unreleased changes under a dated release section and
include the dead letter pane's opt-in live updates and River upgrade.
This prepares the main module for the first phase of the UI and Pro
release process.

Validation: 278 frontend tests, production build, frontend lint,
and Go race tests and lint across all modules.

release riverproui/v0.22.0

Prepare version v0.21.0 (#700)

release riverproui/v0.21.0

Prepare version v0.20.0 (#695)

release riverproui/v0.20.0

Upgrade packager Go version to 1.26 + regression test (#667)

While trying to do a release yesterday, the final release build failed
unfortunately because `packager/go.mod` specified Go 1.25, and that was
too old to run the Go 1.26 found in `./go.mod`.

Here, upgrade the packager to Go 1.26, and also put in a test that'll
detect such a discrepancy in CI and fail on the branch rather than
failing only come release time, letting us catch the problem earlier and
avoid having to go back and fix up a release.

release riverproui/v0.19.0

Prepare version v0.18.1 (#664)

Since no River UI changes were made here, I'll release both River UI and
River Pro UI point to the same commit.

Prepare release v0.18.0 (#661)

It's been a while since we've done a UI release, so here bundle up some
changes from the last few months.