This pull request refactors the handling of URL conversions in the codebase, standardizing the approach by using URI.create(...).toURL() instead of direct new url(/Education?url=https%3A%2F%2Fgithub.com%2Fmapstruct%2Fmapstruct%2Fpull%2F...) construction. This change affects both the internal conversion logic and the related tests, aiming for improved consistency and reliability in URL handling.

Refactoring of URL Conversion Logic:

• The ConversionUtils.url method and related imports of java.net.URL were removed, and all references were updated to use URI and the ConversionUtils.uri method instead.
• In URLToStringConversion.java, the conversion now constructs URLs using URI.create(<SOURCE>).toURL() rather than new url(/Education?url=https%3A%2F%2Fgithub.com%2Fmapstruct%2Fmapstruct%2Fpull%2F%26lt%3BSOURCE%26gt%3B). This also updates the import types from URL to URI.

Test Adjustments for New Conversion Approach:

• In URLConversionTest.java, all usages of new url(/Education?url=https%3A%2F%2Fgithub.com%2Fmapstruct%2Fmapstruct%2Fpull%2F...) were replaced with URI.create(...).toURL() to match the new conversion logic.
• Test cases for invalid URL strings were updated to use malformed URI schemes (e.g., "xxxxxx://mapstruct.org/") instead of arbitrary invalid strings, ensuring the tests accurately reflect the new conversion path.

Closes #4041