chore: upgrade kubebuilder from go/v3 to go/v4

[nakamasato]

• https://github.com/kubernetes-sigs/kubebuilder/blob/master/testdata/project-v4
• https://book.kubebuilder.io/migration/manually_migration_guide_gov3_to_gov4

[codecov]

Codecov Report

Merging #259 (870275e) into main (df283c1) will decrease coverage by 0.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #259      +/-   ##
==========================================
- Coverage   60.83%   60.08%   -0.75%     
==========================================
  Files           6        6              
  Lines         669      669              
==========================================
- Hits          407      402       -5     
- Misses        210      214       +4     
- Partials       52       53       +1     

Files Changed	Coverage Δ
internal/controller/mysql_controller.go	53.80% <ø> (ø)
internal/controller/mysqldb_controller.go	36.88% <ø> (ø)
internal/controller/mysqluser_controller.go	64.97% <ø> (ø)
internal/controller/test_utils.go	76.00% <ø> (ø)