Update issue templates

For geneontology/go-site#2406 and geneontology/go-site#2407

.github/ISSUE_TEMPLATE/custom.md

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+---
+name: Custom issue template
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+

.github/ISSUE_TEMPLATE/go-download-data-changes.md

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+---
+name: GO download data changes
+about: Let users know about upcoming changes to GO data downloads
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+What:
+
+Change timeline:
+
+Impacted users:
+
+Data download release notes can be found at:

.github/ISSUE_TEMPLATE/widget-library-release.md

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+---
+name: Widget library release
+about: Announcement of widget library release
+title: GO widget library release (ver. X.Y.Z)
+labels: release
+assignees: ''
+
+---
+
+We are pleased to announce a new release of the GO widget library!
+
+Release notes can be found here:
+
+The most salient changes for this release are: 
+
+If you have any questions or concerns, please let us know.