[26.0] Update version to 26.0.rc1

[guerler]

(Please replace this header with a description of your pull request. Please include BOTH what you did and why you made the changes. The "why" may simply be citing a relevant Galaxy issue.)
(If fixing a bug, please add any relevant error or traceback)
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How to test the changes?

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• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

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