doc: update Bitgesell issue templates

[220nightmore-spec]

Description

Updates the GitHub issue templates so new reports point to Bitgesell/BGL Core resources instead of inherited Bitcoin Core locations.

This keeps the change limited to .github/ISSUE_TEMPLATE files and avoids touching runtime or consensus code.

Notes

• Replaces Bitcoin Core wording in the bug report template with BGL Core wording.
• Points bug reporters to the local SECURITY.md and local doc/files.md instead of external Bitcoin Core pages.
• Updates the good-first-issue template to remove mojibake in the useful-skills examples and link to this repository's CONTRIBUTING.md.
• Updates the GUI issue template so BGL-qt reports stay in the Bitgesell repository instead of being redirected to the Bitcoin Core GUI tracker.

Validation:

• git diff --check
• Checked the edited templates for required issue-template keys (name, description, labels, body) and tab-free indentation.
• Confirmed the edited templates no longer contain Bitcoin Core/bitcoin-core issue tracker links.

BTC/BGL PR reward address

Bounty issue: #39

Preferred payout: USDT ERC20 or BEP20/BSC to 0x185e250a54ced0d999cbc3280ca8481b35aa2ce0