feat: change severity for avoid-banned-imports, prefer-trailing-comma, ban-name rules

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

• Documentation update
• Bug fix
• New rule
• Changes an existing rule
• Add autofixing to a rule
• Add a CLI option
• Add something to the core
• Other, please explain:

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 515

• Total lines of source code: 8887

• Total classes: 388

• Average Cyclomatic Number per line of code: 0.37 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1718.0 hours

• Found issues: 5 ⚠

[codecov]

Codecov Report

Merging #1052 (ccc777f) into master (0f646c4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1052   +/-   ##
=======================================
  Coverage   86.47%   86.47%           
=======================================
  Files         326      326           
  Lines        6933     6933           
=======================================
  Hits         5995     5995           
  Misses        938      938           

Impacted Files	Coverage Δ
...lyzer/rules/rules_list/tag_name/tag_name_rule.dart	100.00% <ø> (ø)
...void_banned_imports/avoid_banned_imports_rule.dart	100.00% <100.00%> (ø)
...lyzer/rules/rules_list/ban_name/ban_name_rule.dart	100.00% <100.00%> (ø)
...fer_trailing_comma/prefer_trailing_comma_rule.dart	100.00% <100.00%> (ø)

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