Improve release issue template

[oleiade]

What?

This PR fixes the grammar of the release checklist, and revamps it a bit to address the unclarities I experienced when using it for the v0.46.0 release.

Why?

The last item of the checklist is about bringing improvements to the checklist if possible, so I've given it a shot.
I found the language of the checklist could be improved to make it easier to read, and I experienced difficulties at times really understanding what I needed to do. Although I believe some further improvements would still be possible.

PS: I've also toyed with a much more concise and direct style, just like in aviation checklists, maybe that's even better?

// ...

## Pre-Release - One Week Out

- [ ] Confirm merge: All k6-docs PRs with new/modified features for this version.
- [ ] Confirm merge: All k6 PRs for this [milestone](https://github.com/grafana/k6/milestones).
- [ ] Open release notes PR. Request contributions from relevant teams (@k6-browser, @k6-chaos, @devrel teams, etc.).
- [ ] Share notes PR in `#k6-oss-dev` Slack. Tag and request contributions from all impacted teams.
- [ ] PR: Bump [k6 Go project version](https://github.com/grafana/k6/blob/9fa50b2d1f259cdccff5cc7bc18a236d31c345ac/lib/consts/consts.go#L11).
- [ ] PR: Update k6 type definitions at `DefinitelyTyped/DefinitelyTyped`.

## Pre-Release - One Day Out

- [ ] PR: Archive k6's JavaScript API in k6-docs as per [instructions](https://github.com/grafana/k6-docs/wiki/Add-version-for-Javascript-API-documentation).
- [ ] Confirm k6-docs PRs for new features/changes are reviewed and updated.

// ...

[codecov-commenter]

Codecov Report

Merging #3275 (54868f0) into master (cbd9e9a) will increase coverage by 0.03%.
Report is 15 commits behind head on master.
The diff coverage is n/a.

❗ Current head 54868f0 differs from pull request most recent head 41efc00. Consider uploading reports for the commit 41efc00 to get more accurate results

@@            Coverage Diff             @@
##           master    #3275      +/-   ##
==========================================
+ Coverage   73.19%   73.22%   +0.03%     
==========================================
  Files         258      258              
  Lines       19884    19884              
==========================================
+ Hits        14555    14561       +6     
+ Misses       4405     4400       -5     
+ Partials      924      923       -1     

Flag	Coverage Δ
ubuntu	73.10% <ø> (-0.05%)	⬇️
windows	73.07% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

see 3 files with indirect coverage changes

[codebien]

LGTM

[mstoykov]

LGTM in general - I mostly disagree with having the docs PR merged.

[olegbespalov]

❤️