Update dependency mkdocs-material to v9.1.15

This MR contains the following updates:

Package	Update	Change
mkdocs-material	patch	9.1.14 -> 9.1.15
mkdocs-material	patch	==9.1.14 -> ==9.1.15

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Release Notes

squidfunk/mkdocs-material

v9.1.15: mkdocs-material-9.1.15

Compare Source

• Fixed #​5566: Indicate color scheme to operating system
• Fixed #​5565: Update Dockerfile to latest version of base image
• Fixed #​5554: Add additional version tags (9, 9.1) to Docker image
• Fixed #​5536: Strip tags of ARIA labels in table of contents

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