Downgrade generated code to target Java 8

[nikita-volkov]

Motivation

The upcoming pgenie-maven-plugin (source-injection model: generated sources are compiled as part of the consumer's own module) targets maximal consumer compatibility. Production JVM stats still show ~29% of applications on Java 8, disproportionately the DB-heavy enterprises pGenie targets. The generated code currently requires JDK 16+ (records), which caps the reachable market regardless of the plugin's own floor.

Scope

Rewrite the generation templates so the emitted code compiles under --release 8:

• records → plain final classes (fields, constructor, accessors, equals/hashCode/toString)
• text blocks (SQL literals) → string concatenation
• any List.of / var / other post-8 idioms → Java 8 equivalents
• generated pom.xml: maven.compiler.source/target = 1.8

Prerequisite

io.codemine.java.postgresql:jdbc (the single compile-scope dependency of generated code) must itself ship Java 8 bytecode. Track that downgrade in its own repo; this issue depends on it.

Acceptance

• The demo project's generated output compiles with --release 8.
• Generated integration tests still work on the chosen JUnit 5 / Testcontainers versions (both support Java 8).

[nikita-volkov]

Prerequisite filed: codemine-io/postgresql-jdbc.java#2 (codemine-io/postgresql-jdbc.java#2)