Add update script to fetch new kn version and update formulas

[dsimansk]

I've done a bit of overhaul on the script we have for manual update.

My idea is to drive the execution through GH actions primarily, although it's runable locally in current format. Create one GH action with manual run dispatch trigger. And periodical trigger on a day after patch release job is executed, give or take simply put once per week.

The logic of fetching version is taken from client and usual git parsing tag approach.

Then generating the file based on template. Also old file is always generated to get rid of sed shenanigans. :)

It can update to current patch version release and bump to new 1.y.z minor every release cycle.

It can't update previous patch versions though. I could add a logic to always check and reconcile N-2 version. But seemed like a corner case to me.

Superseding: https://github.com/knative/homebrew-client/blob/main/hack/update-versions.sh

/cc @rhuss

[dsimansk]

The update action showcase on my fork: dsimansk#1

[dsimansk]

Failed CI check is unrelated to this changes, being addressed by #137.

[rhuss]

Thanks, looks great! Some minor nits inline, as usual ;-)

[dsimansk]

@rhuss updated per review suggestions. Thanks! 46a5bed

[rhuss]

you could mark all those variables as local

[dsimansk]

Well, I had that though, but then shellcheck wanted me to split declaration and I was too lazy to do it. :)

[dsimansk]

I'll change it NP.

[dsimansk]

24dfe8e

[rhuss]

strange, wonder what the reasoning of shellcheck is for insisting of having declaration and initialization in two statements and why it doesn't complain about global variables ;-)

[rhuss]

Looks good to me with one minor nit, but I don't insist on that :-)

/approve
/lgtm
/hold

[dsimansk]

@rhuss updated with local. I've seen that coming from miles away, anyway. :)

/unhold

[rhuss]

/approve
/lgtm

[knative-prow]

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[rhuss]

Thanks again for taking care!