RFR: 8344023: Unnecessary Hashtable usage in LdapClient.defaultBinaryAttrs

[Andrey Turbanov]

Content of Hashtable `com.sun.jndi.ldap.LdapClient#defaultBinaryAttrs` is fully initialized in `<clinit>` block.
It means we can replace legacy synchronized `Hashtable` with immutable set.

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Commit messages:
 - [PATCH] Unnecessary Hashtable usage in LdapClient.defaultBinaryAttrs

Changes: https://git.openjdk.org/jdk/pull/21903/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21903&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344023
  Stats: 29 lines in 1 file changed: 1 ins; 9 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/21903.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21903/head:pull/21903

PR: https://git.openjdk.org/jdk/pull/21903