mvnup: Add HTTP-to-HTTPS repository URL upgrade strategy

[gnodet]

Summary

• Adds a new RepositoryHttpsUpgradeStrategy that automatically converts http:// repository URLs to https:// in POM files
• Since Maven 3.8.1+, HTTP repositories are blocked by the maven-default-http-blocker mirror, so any http:// repository URL causes build failures
• Well-known repositories (Apache snapshots/releases, Maven Central variants like repo1.maven.org, central.maven.org) are mapped to their canonical HTTPS equivalents
• All other http:// URLs get a generic scheme upgrade to https://

Sections handled

• <repositories>/<repository> URLs
• <pluginRepositories>/<pluginRepository> URLs
• <distributionManagement>/<repository> and <snapshotRepository> URLs
• Profile-scoped repositories in all of the above sections

Test plan

• Well-known Apache repository URL normalization (snapshots, releases) — 2 tests
• Maven Central variant normalization (repo1, central, repo.maven.apache.org) — 3 tests
• Trailing slash preservation for well-known URLs — 1 test
• Unknown URL returns null from mapping — 1 test
• Generic http→https conversion — 2 tests
• URLs already using https (no change) — 1 test
• POM with no repositories (no change) — 1 test
• Plugin repository URL upgrade — 1 test
• distributionManagement repository and snapshotRepository — 3 tests
• Profile-scoped repositories and pluginRepositories — 2 tests
• Multiple repositories in a single POM — 2 tests
• Repositories across all sections simultaneously — 1 test
• Edge cases (no URL element, no id element, central.maven.org normalization) — 3 tests
• Applicability tests (--model, --all, default, --model=false) — 4 tests
• All 495 existing tests pass

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