AzureMonitorAgent: add Ubuntu 26.04 to supported distros

[kamilon]

Adds Ubuntu 26.04 to both the x86_64 and aarch64 AMA supported-distro maps.

Companion changes:

• Test orchestrator: https://dev.azure.com/msazure/One/_git/EngSys-MDA-CloudAgent/pullrequest/16638681
• Azure Policy image allow-lists: commit prepared; repository currently blocks direct pushes and new forks

Validation results (AMA 1.42.0, Ubuntu 26.04):

• x86_64 (vm260730232620ubuntu26x8664): Heartbeat 19, Syslog 5,095, Perf 45,244, InsightsMetrics 478, MyTable_CL 17, MyTableAst_CL 16. Heartbeat reported Category Azure Monitor Agent, Version 1.42.0.
• aarch64 (vm260730232620ubuntu26aarch64): Heartbeat 6, Syslog 1,795, Perf 14,474, InsightsMetrics 147, MyTable_CL 5, MyTableAst_CL 5. Heartbeat reported Category Azure Monitor Agent, Version 1.42.0.
• MyTableAst_CL was isolated per VM using _ResourceId because its Computer field is empty; all other tables were isolated using Computer.

Changes required to make logs flow:

No distro-specific changes were required. Canonical's Ubuntu 26.04 images include active rsyslog, and all expected tables flowed after the temporary unsupported-distro gate override used for validation.

[karmafeast]

Thanks for the original Ubuntu 26.04 allowlist change and public x86_64/aarch64 validation.

For reference, #2201 builds on this change with repository regression coverage for:

• the production installer gate on x86_64 and aarch64;
• the troubleshooting gate and support maps on both architectures;
• Ubuntu 26.04 distro detection;
• Python 3.13 and 3.14 CI; and
• preserving the shared support list while formatting unsupported-version guidance.

All six public fork CI jobs pass. If #2199 remains the preferred merge vehicle, the test coverage in #2201 is available for review or reuse.