Replace manual pipe-separated text formatting with Slack's native
table block API for cleaner, more readable reports.

Changes:
- Use table blocks with proper cell types (raw_text, rich_text)
- Jira issues displayed as clickable links in table cells
- Header row: U | Issue | Fix | Aff | Priority | Severity | Deadline | PRs
- Legend moved to section text above table
- PRs and orphaned issues remain as section blocks
- Removed obsolete _split_slack_sections function

Table format per Slack API:
- Headers: First row in rows array
- Cells: raw_text (simple) or rich_text (links)
- Max 100 rows, 20 columns per table

Example table output:
┌─────┬───────────┬─────┬─────┬──────────────────┬──────────┬──────────────┬──────┐
│  U  │   Issue   │ Fix │ Aff │    Priority      │ Severity │   Deadline   │ PRs  │
├─────┼───────────┼─────┼─────┼──────────────────┼──────────┼──────────────┼──────┤
│ 🔴  │ ROX-12345 │ ❌  │ ✅  │ :jira-critical:  │ Important│ Due: 2d      │ #1234│
│ 🟡  │ ROX-12346 │ ✅  │ ❌  │ :jira-major:     │ Moderate │ Due: 12d     │ #1236│
└─────┴───────────┴─────┴─────┴──────────────────┴──────────┴──────────────┴──────┘

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Replace separate "PRs Missing Jira" and "Issues with Missing Metadata"
sections with one unified "Release Contents" table showing everything.

Changes:
- New _create_all_pr_rows() function collects all PRs and issues
- All PRs shown in table (with or without Jira)
- All issues shown (complete and incomplete metadata)
- PRs without Jira show "No Jira" in Issue column
- Problems sorted first (incomplete), then by urgency
- Complete items shown after problems

Table structure:
- Urgency | Issue | fixVersion | affectedVersion | Priority | Severity | Deadline | PRs
- Shows ❌ for missing metadata, ✅ for present
- "No Jira" for PRs without Jira reference
- "—" for N/A fields

Benefits:
- Single comprehensive view of entire release
- Problems highlighted at top (sorted by completeness, then urgency)
- No separate sections to cross-reference
- Easy to see both what's good and what needs attention

Example rows:
- ❌ issue: ROX-123 | ❌ | ✅ | :jira-critical: | Important | Due: 2d | #1234
- ✅ issue: ROX-456 | ✅ | ✅ | :jira-major: | — | — | #1235
- No Jira PR: — | No Jira | — | — | — | — | — | #1236

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>