Remove the retired ModelCompiler object in migrate; bump Testcontainers to 2.0.5

[alexander-yevsyukov]

Two independent buildSrc-distribution fixes.

1. migrate — remove the retired ModelCompiler dependency object

ModelCompiler.kt was dropped from config in cce5f57 ("Remove outdated local
dependency"), but migrate was never updated to delete stale copies in consumer
repositories.

migrate distributes buildSrc with cp -R buildSrc .., which is a pure overlay:
it adds and overwrites, but never deletes. So every file config retires needs an
explicit rm -f, otherwise a consumer that pulled earlier keeps the retired object
forever — and buildSrc compiles as a single Kotlin source set, so a leftover
object can break the consumer's build.

ModelCompiler.kt now joins the existing removal block next to ProtoData.kt and
McJava.kt, and the block comment records the supersession chain:
Model Compiler → McJava → CoreJvm Compiler (io.spine.core-jvm).

2. Testcontainers — bump to 2.0.5 with the 2.x artifact names

Testcontainers 2.x prefixes every module artifact with testcontainers-; only the
core library keeps the plain testcontainers name. Testcontainers.kt now pins
2.0.5 with the 2.x coordinates, so config floats stop reverting consumers that
already upgraded (gcloud-jvm PR #202) and stop breaking their
Testcontainers-2.x-based sources.

Also adds a postgresql constant so config's copy is a superset of the consumer
usages — jdbc-storage/rdbms/build.gradle.kts references it, and without it the
next float breaks that build script with an unresolved reference.

Note the constant names (junitJupiter, gcloud, mySql) are unchanged — only
their coordinate values move — so no consumer reference breaks and no @Deprecated
shim is owed. postgresql intentionally keeps its lowercase spelling rather than
matching mySql's camelCase, because that is the name jdbc-storage already
references.

Verification

• ./gradlew -p buildSrc build passes (JDK 17).
• bash -n migrate passes; the added rm -f path matches the historical file path
  exactly, sits inside the IS_JVM block after cp -R buildSrc .., and rm -f
  keeps re-runs a no-op.
• Testcontainers coordinates verified against Maven Central at 2.0.5:
  testcontainers, testcontainers-junit-jupiter, testcontainers-gcloud,
  testcontainers-mysql, and testcontainers-postgresql all resolve, while the
  old unprefixed junit-jupiter, gcloud, and mysql return 404. 2.0.5 is the
  current release per maven-metadata.xml.
• dependency-audit: APPROVE — no version downgrade, no removed symbols, no
  stale coordinates left in the repo, copyright current.
• This repository has no root version.gradle.kts, so the version gate is not
  applicable.

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