Update issue templates

.github/ISSUE_TEMPLATE/custom.md

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+---
+name: Custom issue template
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- This issue tracker is only for technical issues related to Eclair.
+
+Please do not open issues for support requests or questions about Lightning or Eclair: use Github discussions instead (https://github.com/ACINQ/eclair/discussions).
+
+Please check that there is not already a similar issue before opening a new one.
+-->