Allow numbers in TTS engine part of cache key

[synesthesiam]

TTS engine name is part of cache key, but if the engine (now entity) name contains numbers (such as tts.piper_2) the following happens:

1. The TTS audio is generated, cached, and written to the file system correctly
2. Requests for the cached audio will fail because the non-legacy regex fails to match the engine

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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Additional information

• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @home-assistant/core, @pvizeli, mind taking a look at this pull request as it has been labeled with an integration (tts) you are listed as a code owner for? Thanks!

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