Increment minimum required CMake version to 3.15 (2020)

[martinmoene]

See expected-lite, issue 71.

Plan:

• lest
• active-lite
• any-lite
• atomic-lite
• bit-lite
• boolean-lite
• byte-lite
• endian-lite
• expected-lite
• indirect-value-lite (see also value-ptr-lite)
• invoke-lite
• jthread-lite
• observer-ptr-lite
• optional-bare
• optional-lite
• optional-fun-lite
• ring-span-lite
• scope-lite
• span-lite
• status-value-lite
• string-lite
• string-view-lite
• type-lite
• value-ptr-lite (see also indirect-value-lite)
• variant-lite
• nonstd-lite-project

Other:

• rpc-lite