Docker cross compile - remove MIPS target platform

[starypatyk]

Follow up to #1297 and #1298

Given that:

• MSRV was moved up to 1.73.
• Even with rust 1.71 the MIPS target did not build successfully.
• Apparently there is hardly any interest in maintaining the MIPS platform.
• Last but not least - the Docker image for cross-compilation did not build - since MIPS target was demoted to tier 3 at 1.72.

I propose to remove the MIPS architecture entirely from the Docker image for cross compilation.

After these changes, I was able to:

• Build the Docker image.
• Cross compile for all remaining platforms.
• Verify that x86_64 and aarch64 binaries work fine on Ubuntu 22.04 (regular PC) and Raspberry PI OS (64-bit/bookworm/Raspberry Pi 3) respectively.

[roderickvd]

All good to me. If someone pops up in need of MIPS support, he/she is more than welcome to contribute and maintain that him/herself.

[starypatyk]

I have updated the Cross compiling Wiki page to reflect current status.