Replace StringBuffer with StringBuilder in org.cactoos.text.Replaced (#1410)

[mvanhorn]

Closes #1410.

Replaces StringBuffer with StringBuilder in org.cactoos.text.Replaced. The buffer is scoped to a single lambda invocation and never escapes that thread, so the synchronization StringBuffer inherits from AbstractStringBuilder is wasted work.

Matcher.appendReplacement(StringBuilder, String) and Matcher.appendTail(StringBuilder) have been available since Java 9; the project's pom.xml already targets Java 17, so the StringBuilder overloads compile cleanly.

The existing ReplacedTest covers the public behaviour and stays unchanged.

[GHX5T-SOL]

Verified current head 7fabfcf6054a2b49236c802526d0f40a3a4c2a76.

The change is scoped to the production Replaced buffer used by Matcher.appendReplacement / appendTail. Since pom.xml compiles with Java 17, the StringBuilder overloads are available, and the buffer is local to one mapped string operation. I also checked that this removes the only production StringBuffer occurrence; the remaining StringBuffer references are test coverage for APIs that accept it.

Local validation on this head:

• mvn -q -DskipTests compile -> passed.
• mvn -q -Dtest=ReplacedTest -Djacoco.skip=true -Dhone.skip=true test -> 11 tests, 0 failures, 0 errors, 0 skipped.
• git grep -n "StringBuffer" origin/master -- src/main/java src/test/java vs HEAD -> production Replaced.java occurrence removed; only test uses remain.
• git diff --check origin/master...HEAD -> clean.
• git diff --no-ext-diff origin/master...HEAD | gitleaks stdin --no-banner --redact --exit-code 1 -> no leaks found.

Hosted Maven is currently unstable because the macOS job fails inside hone-maven-plugin with Cannot run program "docker", and the other Maven jobs are cancelled. I don't see that as caused by this one-line Replaced change.