fix: add retry with backoff to get-version calls in version resolution

[mykaul]

Summary

• Add retry logic (5 attempts, linear 10s backoff) to all get-version invocations in resolve-cassandra-version and resolve-scylla-version Makefile targets
• Fixes transient CI failures caused by GitHub API rate limiting (HTTP 403) during version resolution

Problem

The get-version binary queries the GitHub Tags API (for Cassandra) and DockerHub API (for Scylla) to resolve version aliases like 4-LATEST. These APIs can return transient errors — notably HTTP 403 rate limit exceeded on unauthenticated GitHub API requests. With set -eo pipefail, any such failure is immediately fatal, causing the entire CI job to fail.

Example failure:

failed to execute query to https://api.github.com/repos/apache/cassandra/tags?per_page=100,
last error: rate limit exceeded: server replied with 403 rate limit exceeded

Solution

A reusable retry() bash function defined via a Make define block, injected into both version resolution targets. On failure, it retries with 10s/20s/30s/40s/50s delays (5 attempts, ~2.5 min max wait). Key properties:

• stdout/stderr separation: retry progress messages go to stderr, so the version string captured by backtick substitution is never corrupted
• Fail-fast preserved: after all attempts are exhausted, the last error is printed and the function returns non-zero
• Cache bypass: the existing version file cache means successful runs never enter the retry path
• No new dependencies: pure bash, uses the existing .ONESHELL Makefile setup

Testing

Verified with 5 test scenarios:

1. Command succeeds on first try — output captured correctly
2. Command always fails — returns non-zero after max attempts
3. Transient failure (fails 2x, succeeds on 3rd) — the rate-limit scenario
4. stdout/stderr separation — version string not corrupted
5. Pipe compatibility — retry cmd | tr -d '"' works (matches actual Makefile usage)

Also verified via make --dry-run that both resolve-cassandra-version and resolve-scylla-version expand correctly.