chore: remove migrationcli code and relevant makefile code

[Light2Dark]

Description

Remove migrationcli code and any relevant commands as they will no longer work too.

Closes issue(s)

Resolve #2402

Checklist

• I have tested this code - unit tests pass (integrations tests have errors but its expected I think)
• I have added unit test to cover this code
• I have updated the documentation (README.md and /website/docs)
• I have followed the contributing guide

[netlify]

✅ Deploy Preview for go-feature-flag-doc-preview canceled.

Name	Link
🔨 Latest commit	7bf5df7
🔍 Latest deploy log	https://app.netlify.com/sites/go-feature-flag-doc-preview/deploys/6704eaf957222500081974c6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.55%. Comparing base (1bae773) to head (7bf5df7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2457      +/-   ##
==========================================
- Coverage   85.74%   85.55%   -0.20%     
==========================================
  Files         102      100       -2     
  Lines        3606     3523      -83     
==========================================
- Hits         3092     3014      -78     
+ Misses        390      386       -4     
+ Partials      124      123       -1     

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[thomaspoignant]

Thanks for this PR.
I will merge it a bit later after the issue #2401 will be merged.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud