Fix release script prerelease GitHub template release

[ivanvc]

Follow up on #19334:

• Properly get the minor version from prereleases (i.e., v3.6.0-rc.0 → v3.6, not v3.6.0-rc). I tested it on my fork, and the linked pages are as expected (note that we still have a broken link in the upgrade guide): https://github.com/ivanvc/etcd/releases/tag/v3.6.0-rc.0
• Fix a typo in "flag". It said "flat".

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.91%. Comparing base (8c52b41) to head (5c9db9c).
Report is 3 commits behind head on main.

Additional details and impacted files

see 23 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19418      +/-   ##
==========================================
- Coverage   68.97%   68.91%   -0.07%     
==========================================
  Files         420      420              
  Lines       35753    35753              
==========================================
- Hits        24662    24639      -23     
- Misses       9664     9689      +25     
+ Partials     1427     1425       -2     

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[serathius]

scripts/release.sh:367:11: warning: Declare and assign separately to avoid masking return values. [SC2155]

[ivanvc]

scripts/release.sh:367:11: warning: Declare and assign separately to avoid masking return values. [SC2155]

Done. I pushed and went straight to my bed 🛌 😴 😄

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ivanvc, serathius

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[ahrtr]

Do we need to backport this PR to 3.6/3.5/3.4?

[ivanvc]

Do we need to backport this PR to 3.6/3.5/3.4?

We need it in 3.6. We can do 3.5 and 3.4 for consistency, but it is not needed.

/cherry-pick release-3.6

[k8s-infra-cherrypick-robot]

@ivanvc: new pull request created: #19420

In response to this:

Do we need to backport this PR to 3.6/3.5/3.4?

We need it in 3.6. We can do 3.5 and 3.4 for consistency, but it is not needed.

/cherry-pick release-3.6

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