[Bug] Version 0.19.0 not available through 'az --upgrade' flag

**Describe the bug**
"az extension add --name azure-devops --upgrade" doesn't update to latest 0.19.0 version

**To Reproduce**
Azure Cli Version: azure-cli 2.24.2
Azure-Devops extension version: azure-devops 0.18.0

Steps to reproduce the behavior:
1. az extension add --name azure-devops --upgrade
2.
```
Extension 'azure-devops' 0.18.0 is already installed.
It will be updated if available.
No updates available for 'azure-devops'. Use --debug for more information.
```

**Expected behavior**
I expect the 0.19.0 version to be found and installed

**Debug logs**
```
cli.knack.cli: Command arguments: ['extension', 'add', '--name', 'azure-devops', '--upgrade', '--debug']
cli.knack.cli: __init__ debug log:
Enable color in terminal.
cli.knack.cli: Event: Cli.PreExecute []
cli.knack.cli: Event: CommandParser.OnGlobalArgumentsCreate [, , ]
cli.knack.cli: Event: CommandInvoker.OnPreCommandTableCreate []
cli.azure.cli.core: Modules found from index for 'extension': ['azure.cli.command_modules.extension']
cli.azure.cli.core: Loading command modules:
cli.azure.cli.core: Name Load Time Groups Commands
cli.azure.cli.core: extension 0.001 1 7
cli.azure.cli.core: Total (1) 0.001 1 7
cli.azure.cli.core: These extensions are not installed and will be skipped: ['azext_ai_examples', 'azext_next']
cli.azure.cli.core: Loading extensions:
cli.azure.cli.core: Name Load Time Groups Commands Directory
cli.azure.cli.core: Total (0) 0.000 0 0
cli.azure.cli.core: Loaded 1 groups, 7 commands.
cli.azure.cli.core: Found a match in the command table.
cli.azure.cli.core: Raw command : extension add
cli.azure.cli.core: Command table: extension add
cli.knack.cli: Event: CommandInvoker.OnPreCommandTableTruncate []
cli.azure.cli.core.azlogging: metadata file logging enabled - writing logs to '/home/.azure/commands/2021-06-16.14-23-48.extension_add.138297.log'.
az_command_data_logger: command args: extension add --name {} --upgrade --debug
cli.knack.cli: Event: CommandInvoker.OnPreArgumentLoad [.add_subscription_parameter at 0x7f1cd2e26790>, .register_query_examples at 0x7f1cd2d7f9d0>]
cli.knack.cli: Event: CommandInvoker.OnPostArgumentLoad []
cli.knack.cli: Event: CommandInvoker.OnPostCommandTableCreate [.add_ids_arguments at 0x7f1cd2d7fa60>, .add_cache_arguments at 0x7f1cd2d7fb80>]
cli.knack.cli: Event: CommandInvoker.OnCommandTableLoaded []
cli.knack.cli: Event: CommandInvoker.OnPreParseArgs []
cli.knack.cli: Event: CommandInvoker.OnPostParseArgs [, , .handle_example_parameter at 0x7f1cd2d7f940>, .parse_ids_arguments at 0x7f1cd2d7faf0>]
cli.azure.cli.core.extension.operations: Extension 'azure-devops' 0.18.0 is already installed.
cli.azure.cli.core.extension.operations: It will be updated if available.
urllib3.connectionpool: Starting new HTTPS connection (1): aka.ms:443
urllib3.connectionpool: https://aka.ms:443 "GET /azure-cli-extension-index-v1 HTTP/1.1" 301 0
urllib3.connectionpool: Starting new HTTPS connection (1): azcliextensionsync.blob.core.windows.net:443
urllib3.connectionpool: https://azcliextensionsync.blob.core.windows.net:443 "GET /index1/index.json HTTP/1.1" 200 1008584
cli.azure.cli.core.extension._resolve: Candidates ['azure_devops-0.12.0-py2.py3-none-any.whl', 'azure_devops-0.17.0-py2.py3-none-any.whl', 'azure_devops-0.18.0-py2.py3-none-any.whl']
cli.azure.cli.core.extension._resolve: Candidates ['azure_devops-0.12.0-py2.py3-none-any.whl', 'azure_devops-0.17.0-py2.py3-none-any.whl', 'azure_devops-0.18.0-py2.py3-none-any.whl']
cli.azure.cli.core.extension._resolve: Candidates ['azure_devops-0.12.0-py2.py3-none-any.whl', 'azure_devops-0.17.0-py2.py3-none-any.whl', 'azure_devops-0.18.0-py2.py3-none-any.whl']
cli.azure.cli.core.extension._resolve: Skipping 'azure_devops-0.12.0-py2.py3-none-any.whl' as 0.12.0 not greater than current version 0.18.0
cli.azure.cli.core.extension._resolve: Skipping 'azure_devops-0.17.0-py2.py3-none-any.whl' as 0.17.0 not greater than current version 0.18.0
cli.azure.cli.core.extension._resolve: Skipping 'azure_devops-0.18.0-py2.py3-none-any.whl' as 0.18.0 not greater than current version 0.18.0
cli.azure.cli.core.extension.operations: No suitable extensions found.
cli.azure.cli.core.extension.operations: No updates available for 'azure-devops'. Use --debug for more information.
cli.knack.cli: Event: CommandInvoker.OnTransformResult [, ]
cli.knack.cli: Event: CommandInvoker.OnFilterResult []
cli.knack.cli: Event: Cli.SuccessfulExecute []
cli.knack.cli: Event: Cli.PostExecute []
az_command_data_logger: exit code: 0
cli.__main__: Command ran in 1.830 seconds (init: 0.071, invoke: 1.759)
telemetry.save: Save telemetry record of length 3071 in cache
telemetry.check: Negative: The /home/.azure/telemetry.txt was modified at 2021-06-16 14:19:57.147717, which in less than 600.000000 s
```

**Additional context**
Running in WSL2

Reproduce with `az extension add --name azure-devops --upgrade --debug` using the reported Azure CLI and extension versions. Trace the extension index request and `cli.azure.cli.core.extension._resolve` candidates, then verify that version 0.19.0 is discovered and installed instead of reporting no updates.

Written by the indexing model from the issue text.