RFR: 8343418: Use HashMap instead of Hashtable for CSS.htmlAttrToCssAttrMap

[Andrey Turbanov]

If a thread-safe implementation is not needed, it is recommended to use HashMap instead of legacy synchronized Hashtable. 
Map `CSS.htmlAttrToCssAttrMap` is modified only within static initialization block and then only `get` method is called.

The only subtle difference is that Hashtable doesn't allow `null` keys and throws NPE from `get` method.
I've checked all possible keys which are passed to `htmlAttrToCssAttrMap.get`.
Currently 3 different execution paths are possible:

**1,2**
When `HTML.Attribute.BORDER` is passed as a key. It's definitely non-null.

javax.swing.text.html.CSS#translateHTMLToCSS
  translateAttribute(HTML.Attribute.BORDER, "1", cssAttrSet);
    CSS.Attribute[] cssAttrList = getCssAttribute(key);


javax.swing.text.html.CSS#translateAttributes
   translateAttribute(HTML.Attribute.BORDER, Integer.toString(borderWidth), cssAttrSet);
     CSS.Attribute[] cssAttrList = getCssAttribute(key);`


**3**
When non-null `key` is passed as a key.

javax.swing.text.html.CSS#translateAttributes

    if (name instanceof HTML.Attribute) { // from this `instanceof` we know that it's non-null
         HTML.Attribute key = (HTML.Attribute)name;

      translateAttribute(key, (String) htmlAttrSet.getAttribute(key), cssAttrSet);
        CSS.Attribute[] cssAttrList = getCssAttribute(key);


![idea_analyze_dataflow_to_here](https://github.com/user-attachments/assets/48ace4de-6d0a-468e-bb14-b579a193254b)


I've used HashMap.newHashMap method instead of constructor to avoid resizing of internal HashMap array.

-------------

Commit messages:
 - [PATCH] Use HashMap instead of Hashtable for CSS.htmlAttrToCssAttrMap

Changes: https://git.openjdk.org/jdk/pull/21785/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343418
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21785/head:pull/21785

PR: https://git.openjdk.org/jdk/pull/21785