feat: implement maxConcurrentReconciles

[armru]

Release Notes

The operator now supports the maxConcurrentReconciles parameter, with the exception of backups. [[WE SHOULD ADD A LINK TO THE DOC HERE]].

Closes #5687

Note for the reviewers

• if we agree on the feature we should add the proper documentation
• careful testing is needed
• backups can go wrong in many ways so this patch does not address that, if we want to add the support it should be done in a different PR and we should avoid starting multiple backups for the same cluster

[armru]

/test limit=local

[github-actions[bot]]

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/11101722194

[armru]

/test limit=local

[github-actions[bot]]

@armru, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/11104754308

[gbartolini]

/test tl=4

[github-actions[bot]]

@gbartolini, here's the link to the E2E on CNPG workflow run: https://github.com/cloudnative-pg/cloudnative-pg/actions/runs/12185922548