#8238 Save dialog is not properly displayed in certain cases

[allyoucanmap]

Description

Fix zIndex value for unsaved map confirmation dialog.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Issue

#8238

What is the new behavior?
Works as expected

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• Yes, and I documented them in migration notes
• No

Other useful information

Reproducible only on https://qa-mapstore.geosolutionsgroup.com/mapstore/#/

[tdipisa]

@ElenaGallo please test in QA.