Replace gitlab templates ReleasePrep label references with Apps::Type::ReleasePreparation

At this moment all the "[ ] Link to the appropriate Release Prep issue" checkboxes in our gitlab issue and MR templates have their helpful link to the currently open relprep issues list broken because we've deprecated the Release Prep label in favor of Apps::Type::ReleasePreparation without updating the URLs.

Fixing them.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information