Update issue templates

Author: martinus

Diff for: .github/ISSUE_TEMPLATE/robin-hood-hashing-is-deprecated.md

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+---
+name: robin-hood-hashing is deprecated
+about: Describe this issue template's purpose here.
+title: 'WARNING: robin-hood-hashing is DEPRECATED'
+labels: ''
+assignees: ''
+
+---
+
+# WARNING: robin-hood-hashing is DEPRECATED.
+
+## I will not add any more features or fix any warnings. Only critical PRs will be merged, if you are willing to write one yourself.
+
+Unfortunately I do not have time to continue development for this hashmap. I have a worthy successor though, please head over to: `ankerl::unordered_dense::{map, set}` https://github.com/martinus/unordered_dense
+
+If you need a very fast unordered map for lookups, you might want to have a look at `boost::unordered_flat_map`.