Make class abstract doesn't preserve modifiers

[Youssef1313]

Version Used:

main

Steps to Reproduce:

Apply make abstract codefix.

partial class C
{
}

partial class C
{
    protected abstract void M();
}

Expected Behavior:

abstract is added and partial is kept.

partial class C
{
}

partial abstract class C
{
    protected abstract void M();
}

Actual Behavior:

partial is removed

partial class C
{
}

abstract class C
{
    protected abstract void M();
}

Note: Fix could be similar to #54216.