Documentation: Fix broken link of feature proposal template in issue_workflow.md

What does this MR do?

Fix broken link of feature proposal issue template in doc/development/contributing/issue_workflow.md.

• Old link: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab/issue_templates/Feature%20Proposal.md
• New link: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.gitlab/issue_templates/Feature%20proposal.md

Related issues

None

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards