add fips release handlers; stop setting dest channel

[qrkourier]

No description provided.

[qrkourier]

This PR needs GH Settings changes before checks can succeed:

1. Set org Secret secrets.ZHOOK_URL_DEV_NOTIFICATIONS to the value provided by @qrkourier (to be recommended to users in lieu of generically-named ZHOOK_URL)
2. Set org Secret secrets.ZHOOK_URL_GITHUB_SIG_CORE to the value provided by @qrkourier (to be used by use channel-specific mattermost incoming webhook urls sdk-golang#774)
3. Set org Secret secrets.ZHOOK_URL to the value provided by @qrkourier (causes existing users to send webhooks to a Ziti-only intercept for the dev-notifications default channel)
4. Set org Secret secrets.ZITI_MATTERMOST_IDENTITY to the HA-ready identity JSON provided by @qrkourier (this will cause webhooks to resume flowing via Ziti instead of the public address)

Note for users of this Action: the URL path part is the same as the old ZHOOK_URL, which is probably an org Secret, but that Mattermost Incoming Webhook has been reconfigured to lock it to a single channel, hence the change of name used by this Action's self-test workflow. GitHub repos that consume this Action may continue to use whichever env vars or GH Secrets they wish as inputs to the Action, but know that any which are using the value of org Secret ZHOOK_URL will be unable to override the destination channel by setting the channel input of the Action due to the aforementioned change locking that Incoming Webhook to a specific channel on the Mattermost server-side.

[dovholuknf]

anything 'fips-related' doesn't feel like it really fits in here to me.