Fix xml toString() error for modified name space attributes

[HindujaB]

Purpose

$subject

Fixes #40373
Fixes #41521

Approach

• Fix checking 'xmlns` as the default NS key when writing XML value with default namespace attributes.
• Fixed wrong class casting for whitespace values at the start of the XML query
• Removed disableOnOldParser test group as it is no-longer used

Samples

import ballerina/io;

xmlns "http://sample.com/wso2/c1";

public function main() {
    xml x1 = xml `<root xmlns:ns3="http://sample.com/wso2/f"></root>`;
    var xAttr = let var x2 = <'xml:Element>x1 in x2.getAttributes();
    //adding attribute with default namespace
    xAttr["{http://sample.com/wso2/c1}foo1"] = "bar1";
    io:println(x1);
}

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[rdulmina]

Needs to enable XMLAttributesTest.testAddAttributeWithQName_5, XMLAttributesTest.testAddNamespaceAsAttribute1 tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b20eaf9) 76.69% compared to head (426f1ac) 76.70%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #41541   +/-   ##
=========================================
  Coverage     76.69%   76.70%           
  Complexity    52669    52669           
=========================================
  Files          2881     2881           
  Lines        198537   198537           
  Branches      25812    25812           
=========================================
+ Hits         152273   152279    +6     
+ Misses        37832    37825    -7     
- Partials       8432     8433    +1     

Files	Coverage Δ
...erina/runtime/internal/BallerinaXmlSerializer.java	79.24% <100.00%> (-0.95%)	⬇️
.../io/ballerina/runtime/internal/values/XmlItem.java	59.60% <100.00%> (+3.92%)	⬆️

... and 2 files with indirect coverage changes

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[warunalakshitha]

LGTM

[rdulmina]

Let's send this to 8.x as well