chore(deps): update dependency ubuntu to v26

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
ubuntu	github-runner	major	24.04 → 26.04

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[github-actions]

Firmware footprint delta — nrf9151dk/nrf9151/ns

softsim_external_profile app image, from the ELF program headers (flash = load image incl. initialized data, RAM = static SRAM use incl. .bss/noinit):

Memory	base	PR	Δ
Flash	146.1 KiB	146.1 KiB	+0 B
RAM	72.3 KiB	72.3 KiB	+0 B

Section size changes (from the section headers)

(no section size changes)

Top symbol changes (10 largest, from the symbol tables)

(no symbol size changes)

_{Soft report — informational only. Base 418e66207ab626f4a312fdcf6d5656eb97d5c712 vs PR head.}