Revise HttpHeaders Usage

[rwinch]

Spring Framework 7's HttpHeaders no longer implements MultiValueMap and we will need to adjust our usage

If possible, we might be able to make any necessary adjustments in Spring Security 6.5. Otherwise, this needs to happen in Spring Security 7

[ronodhirSoumik]

I explored the related updates for the map interface removal in spring framework v7 code end and also listed down the related refactoring needed in spring-security. I would like to work on this and add a PR.