Add subscript to issue templates for community guidance

[Copilot]

Added community guidance subscripts to both GitHub issue templates to help users understand how to prioritize issues effectively.

Changes Made

• Added dropdown subscript to .github/ISSUE_TEMPLATE/BUG-REPORT.yml
• Added dropdown subscript to .github/ISSUE_TEMPLATE/FEATURE-REQUEST.yml

What the Subscript Does

The subscript appears at the bottom of each issue template as a dropdown element that displays:

• Instructions to use 👍 reactions to help prioritize issues
• Guidance to provide useful context in comments rather than "+1" or "me too"
• Link to OpenTelemetry community resources for issue participation

This follows the standard pattern used across OpenTelemetry repositories to encourage better community participation and issue triage.

Template Structure

Both templates now include the following dropdown element at the end:

- type: dropdown
  attributes:
    label: Tip
    description: This element is static, used to render a helpful sub-heading for end-users and community members to help prioritize issues. Please leave as is.
    options:
      - <sub>[React](https://github.blog/news-insights/product-news/add-reactions-to-pull-requests-issues-and-comments/) with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding `+1` or `me too`, to help us triage it. Learn more [here](https://opentelemetry.io/community/end-user/issue-participation/).</sub>
    default: 0

The YAML syntax has been validated and both templates maintain their existing functionality while adding the community guidance.

Fixes #23.

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