Ensure upgrade tests do not test past the specified version

[desrosj]

Supersedes #10099.

Currently, if you manually dispatch a run of the upgrade testing workflow with a version that is lower than the currently supported major version, the tests run past the specified version instead of stopping when appropriate.

For example, this run was dispatched for version 6.6.4. The jobs spawned included "Upgrading 6.7 to 6.6.4" and "Upgrading 6.8 to 6.6.4)" despite 6.6.4 being older than those two major versions.

The logic does not account for this and no upgrade is actually performed (technically this is a scenario where a downgrade would be tested) because the --force argument is not passed to the wp core update command.

This PR adds consideration for this scenario through a new job that generates a list of exclusions for all following strategy matrices.

• Example passing a minor version (6.4.1)
• Example passing a major version with trailing .0 (6.0.0)
• Example passing a major version without trailing .0 (5.9)
• Example passing a minor version with v appended (v5.5.10)
• Example passing a major version with v appended and trailing .0 (v5.7.0)
• Example passing a major version with v appended and no trailing .0 (v5.2)

Trac ticket:

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