Fix links in the release-checklist template

[oxarbitrage]

https://github.com/ZcashFoundation/zebra/blob/main/.github/PULL_REQUEST_TEMPLATE/release-checklist.md

Almost all links are broken, for example explorer link does not work anymore, several github action links that were moved or deleted, etc.

Will be good to fix this before the next release.

[mpguerra]

Let's do this as part of the next release. @upbqdn can you please take care of it as you'll be tagging the next release?