orthanc-explorer2: Fix IsExistingFile API compatibility

[avinesh2101]

orthanc-explorer2: Fix IsExistingFile API compatibility

Fixes: #71150

RCA : Related: Plugin code uses deprecated API IsExistingFile() which was removed from the Orthanc framework. The method was called in 3 locations (lines 299, 309, 329) to check if custom CSS/logo/favicon files exist.

Fix : Replace with boost::filesystem::exists(), which is already available as a build dependency (boost-dev) and provides identical functionality.

Implementation: Added sed command in the build pipeline to replace all occurrences before compilation. Used sed instead of patch for a simple find-replace operation with no context dependencies.

Pre-review Checklist

For new package PRs only

• [ ] This PR is marked as fixing a pre-existing package request bug
  • [ ] Alternatively, the PR is marked as related to a pre-existing package request bug, such as a dependency
• [ ] REQUIRED - The package is available under an OSI-approved or FSF-approved license
• [ ] REQUIRED - The version of the package is still receiving security updates
• [ ] This PR links to the upstream project's support policy (e.g. endoflife.date)

For new version streams

• [ ] The upstream project actually supports multiple concurrent versions.
• [ ] Any subpackages include the version string in their package name (e.g. name: ${{package.name}}-compat)
• [ ] The package (and subpackages) provides: logical unversioned forms of the package (e.g. nodejs, nodejs-lts)
• [ ] If non-streamed package names no longer built, open PR to withdraw them (see WITHDRAWING PACKAGES)