Upgrade to Spring Boot 4.1.1 and Java 17, fix CI workflow

[Julien Dubois (jdubois)]

Upgrades the sample from Spring Boot 2.7.1 to 4.1.1 and repairs the JUnit workflow, which had been failing on master.

• pom: parent 2.7.1 -> 4.1.1 (Java 17 baseline)
• pom: r2dbc-postgresql groupId io.r2dbc -> org.postgresql (moved at 1.x)
• pom: drop hardcoded h2 / r2dbc-h2 / r2dbc-postgresql versions so the Spring Boot BOM manages them (h2 now 2.4.240, superseding Bump h2 from 2.1.214 to 2.2.220 #4)
• pom: drop junit-vintage-engine exclusion, no longer shipped
• DemoApplication: ConnectionFactoryInitializer and ResourceDatabasePopulator moved to org.springframework.r2dbc.connection.init
• TodoRepositoryTest: DatabaseClient moved to org.springframework.r2dbc.core, and execute() was renamed to sql()
• workflow: bump checkout to v7.0.1 and setup-java to v6.0.0 (from Bump the github-actions group with 2 updates #7), add the distribution input that setup-java v2+ requires, and move to Java 17 to match the pom. Without distribution the build fails with "distribution input is required".

Verified: 5/5 tests pass, and the packaged jar was run against a real PostgreSQL 16 container (POST / -> 201, GET / -> 200, row persisted).

Purpose

• ...

Does this introduce a breaking change?

[ ] Yes
[ ] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

Verify that the following are valid

• ...

Other Information