Update TabMaster to v2.15.1

[Tormak9970]

Update TabMaster to v2.15.1

Literally a single character change because Valve shifted the index of the function we use to calculate the number of games in a tab. Full Changelog: https://github.com/Tormak9970/TabMaster/releases/tag/v2.15.1

Pretty urgent update as its blocking users from using their library

Task Checklist

Developer

• I am the original author or an authorized maintainer of this plugin.
• I have abided by the licenses of the libraries I am utilizing, including attaching license notices where appropriate.
• Generative AI was NOT used to write a majority of the code I am submitting.

Plugin

• I have verified that my plugin works properly on the Stable and Beta update channels of SteamOS.
• I have verified my plugin is unique or provides more/alternative functionality to a plugin already on the store.

Backend

• No: I am using a custom backend other than Python.
• No: I am using a tool or software from a 3rd party FOSS project that does not have it's dependencies statically linked.
• No: I am using a custom binary that has all of it's dependencies statically linked.

Community

• I have tested and left feedback on two other pull requests for new or updating plugins.
• I have commented links to my testing report in this PR.

Testing

• Tested by a third party on SteamOS Stable or Beta update channel.

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[Tormak9970]

Looks like the git branch for my fork got a bit messed up and is showing the previous two tabmaster updates as well, I can try to re-sync it if needed

[Tormak9970]

PR Review 1: #1015 (comment)

currently there are no other approved builds so I can't test others