[INFRA] Consider sharing all Issues and PR templates in the `.github` repository

[tbouffard]

Current situation

We have templates in this repository and also in the .github repository (to share the configuration for all repositories of the organization except this repository). The content is duplicated and not accurate.

We have new needs to better guide people willing to contribute. For example, see

• 2810

• 2811

• 2872

• 2882

Needs

Can we have a single set of templates for all repositories to reduce the maintenance?

Review if we can share more templates and if using specific templates is required in this repository.

### Tasks
- [x] Decide if we can only used shared templates --> No, the templates in the bpmn-visualization repository are too specific
- [x] ~If so, remove the templates from this repository and update the templates of the `.github` repository~ No, keep templates in the bpmn-visualization repository
- [x] If not, apply the recent improvements done in the bpmn-visualization repository to the `.github` repository --> https://github.com/process-analytics/.github/pull/33
- [x] Remove templates from https://github.com/process-analytics/mxgraph-sketch (duplicated from the `.github` repository) --> https://github.com/process-analytics/mxgraph-sketch/pull/89

[tbouffard]

All tasks are completed so closing this issue.