Broken Windows path regex in FilteringUtils.getRelativeFilePath()

In `src/main/java/org/apache/maven/shared/filtering/FilteringUtils.java`, lines 184 and 187:

```
if (toPath.matches("^\\\\\\[a-zA-Z]:")) {
toPath = toPath.substring(1);
}
if (fromPath.matches("^\\\\\\[a-zA-Z]:")) {
fromPath = fromPath.substring(1);
}
```

The regex `^\\\\[a-zA-Z]:` has a double-escaped backslash before the character class `[a-zA-Z]`, making `\\[` match a literal `[` character at the start. The intended regex is `^[a-zA-Z]:` — a drive letter followed by a colon at the start of the path. This regex will never match a Windows absolute path like `C:\\foo`, so the leading-slash stripping logic never executes. This is effectively dead code.

On Windows this could cause incorrect normalization of paths like `/C:/foo` where the leading `/` should be stripped.

Start in src/main/java/org/apache/maven/shared/filtering/FilteringUtils.java at getRelativeFilePath(), especially lines 184 and 187, and trace the handling of a Windows-style /C:/foo path. Correct the two checks so the leading slash is removed, then verify the relevant path-normalization behavior with the existing tests.

Written by the indexing model from the issue text.